Remove skia iOS benchmark - #195180

### Issue Link:
#195180 (PR, there was no open issue)

### Impact Description:
What is the impact (ex. visual jank on Samsung phones, app crash, cannot ship an iOS app)?
Does it impact development (ex. flutter doctor crashes when Android Studio is installed),
or the shipping of production apps (the app crashes on launch).
This information is for domain experts and release engineers to understand the consequences of saying yes or no to the cherry pick.

Release builders should match the master branch configs. new_gallery_skia_ios__transition_perf was removed in #189377. Without this change, the beta release branch will try to run the old builder names and fail because the infra config is missing.

### Changelog Description:
Explain this cherry pick:
* In one line that is accessible to most Flutter developers.
* That describes the state prior to the fix.
* That includes which platforms are impacted.
See [best practices](https://github.com/flutter/flutter/blob/main/docs/releases/Hotfix-Documentation-Best-Practices.md) for examples.

[flutter/195180] Remove obsolete iOS Skia performance benchmark.

### Workaround:
Is there a workaround for this issue?

N/A

### Risk:
What is the risk level of this cherry-pick?

### Test Coverage:
Are you confident that your fix is well-tested by automated tests?

### Validation Steps:
What are the steps to validate that this fix works?

Release PRs should not complain about missing new_gallery_skia_ios__transition_perf builder.

This pull request is created by [automatic cherry pick workflow](https://github.com/flutter/flutter/blob/main/docs/releases/Flutter-Cherrypick-Process.md#automatically-creates-a-cherry-pick-request)
Please fill in the form below, and a flutter domain expert will evaluate this cherry pick request.

### Issue Link:
#195180 (PR, there was no open issue)

### Impact Description:
What is the impact (ex. visual jank on Samsung phones, app crash, cannot ship an iOS app)?
Does it impact development (ex. flutter doctor crashes when Android Studio is installed),
or the shipping of production apps (the app crashes on launch).
This information is for domain experts and release engineers to understand the consequences of saying yes or no to the cherry pick.

Release builders should match the master branch configs. new_gallery_skia_ios__transition_perf was removed in #189377. Without this change, the beta release branch will try to run the old builder names and fail because the infra config is missing.

### Changelog Description:
Explain this cherry pick:
* In one line that is accessible to most Flutter developers.
* That describes the state prior to the fix.
* That includes which platforms are impacted.
See [best practices](https://github.com/flutter/flutter/blob/main/docs/releases/Hotfix-Documentation-Best-Practices.md) for examples.

[flutter/195180] Remove obsolete iOS Skia performance benchmark.

### Workaround:
Is there a workaround for this issue?

N/A

### Risk:
What is the risk level of this cherry-pick?

### Test Coverage:
Are you confident that your fix is well-tested by automated tests?

### Validation Steps:
What are the steps to validate that this fix works?

Release PRs should not complain about missing new_gallery_skia_ios__transition_perf builder.

Impeller opt-out for iOS was removed a long time ago.
`new_gallery_skia_ios__transition_perf` and
`new_gallery_ios__transition_perf` were testing the same things. Remove
the skia variant.

Also `very_long_picture_scrolling_perf_ios__e2e_summary` was passing in
`impeller: false` which is ignored on iOS. Swap the impeller to true.
This doesn't change behavior but now isn't lying about what it's
testing.

This will need a cherry-pick to beta so it doesn't fail in release.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
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- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant in-code documentation (doc comments with
`///`).
- [x] If this PR introduces a new feature or capability, I created and
linked a website documentation issue or PR in [flutter/website] (or
verified none is needed).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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### Issue Link:
flutter/flutter#195180 (PR, there was no open issue)

### Impact Description:
What is the impact (ex. visual jank on Samsung phones, app crash, cannot ship an iOS app)?
Does it impact development (ex. flutter doctor crashes when Android Studio is installed),
or the shipping of production apps (the app crashes on launch).
This information is for domain experts and release engineers to understand the consequences of saying yes or no to the cherry pick.

Release builders should match the master branch configs. new_gallery_skia_ios__transition_perf was removed in flutter/flutter#189377. Without this change, the beta release branch will try to run the old builder names and fail because the infra config is missing.

### Changelog Description:
Explain this cherry pick:
* In one line that is accessible to most Flutter developers.
* That describes the state prior to the fix.
* That includes which platforms are impacted.
See [best practices](https://github.com/flutter/flutter/blob/main/docs/releases/Hotfix-Documentation-Best-Practices.md) for examples.

[flutter/195180] Remove obsolete iOS Skia performance benchmark.

### Workaround:
Is there a workaround for this issue?

N/A

### Risk:
What is the risk level of this cherry-pick?

### Test Coverage:
Are you confident that your fix is well-tested by automated tests?

### Validation Steps:
What are the steps to validate that this fix works?

Release PRs should not complain about missing new_gallery_skia_ios__transition_perf builder.

### Issue Link:
flutter/flutter#195180 (PR, there was no open issue)

### Impact Description:
What is the impact (ex. visual jank on Samsung phones, app crash, cannot ship an iOS app)?
Does it impact development (ex. flutter doctor crashes when Android Studio is installed),
or the shipping of production apps (the app crashes on launch).
This information is for domain experts and release engineers to understand the consequences of saying yes or no to the cherry pick.

Release builders should match the master branch configs. new_gallery_skia_ios__transition_perf was removed in flutter/flutter#189377. Without this change, the beta release branch will try to run the old builder names and fail because the infra config is missing.

### Changelog Description:
Explain this cherry pick:
* In one line that is accessible to most Flutter developers.
* That describes the state prior to the fix.
* That includes which platforms are impacted.
See [best practices](https://github.com/flutter/flutter/blob/main/docs/releases/Hotfix-Documentation-Best-Practices.md) for examples.

[flutter/195180] Remove obsolete iOS Skia performance benchmark.

### Workaround:
Is there a workaround for this issue?

N/A

### Risk:
What is the risk level of this cherry-pick?

### Test Coverage:
Are you confident that your fix is well-tested by automated tests?

### Validation Steps:
What are the steps to validate that this fix works?

Release PRs should not complain about missing new_gallery_skia_ios__transition_perf builder.

### Issue Link:
flutter/flutter#195180 (PR, there was no open issue)

### Impact Description:
What is the impact (ex. visual jank on Samsung phones, app crash, cannot ship an iOS app)?
Does it impact development (ex. flutter doctor crashes when Android Studio is installed),
or the shipping of production apps (the app crashes on launch).
This information is for domain experts and release engineers to understand the consequences of saying yes or no to the cherry pick.

Release builders should match the master branch configs. new_gallery_skia_ios__transition_perf was removed in flutter/flutter#189377. Without this change, the beta release branch will try to run the old builder names and fail because the infra config is missing.

### Changelog Description:
Explain this cherry pick:
* In one line that is accessible to most Flutter developers.
* That describes the state prior to the fix.
* That includes which platforms are impacted.
See [best practices](https://github.com/flutter/flutter/blob/main/docs/releases/Hotfix-Documentation-Best-Practices.md) for examples.

[flutter/195180] Remove obsolete iOS Skia performance benchmark.

### Workaround:
Is there a workaround for this issue?

N/A

### Risk:
What is the risk level of this cherry-pick?

### Test Coverage:
Are you confident that your fix is well-tested by automated tests?

### Validation Steps:
What are the steps to validate that this fix works?

Release PRs should not complain about missing new_gallery_skia_ios__transition_perf builder.