[CP-stable]Remove skia iOS benchmark - #195373

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.

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