[cleanup] Remove orphaned screenshot test files from devicelab #190697

In PR #115848, package:image was removed, which involved deleting the screenshot_test.dart driver script and the integration_ui_screenshot / integration_ui_ios_screenshot targets from .ci.yaml.

However, several scaffolding files and scripts were left behind in the repository. Since the underlying test driver no longer exists and these are no longer running in CI, they are dead code and should be removed to reduce confusion.

Files to delete:

1. dev/devicelab/bin/tasks/integration_ui_screenshot.dart
2. dev/devicelab/bin/tasks/integration_ui_ios_screenshot.dart
3. dev/integration_tests/ui/lib/screenshot.dart (The orphaned app entrypoint since test_driver/screenshot_test.dart was deleted).

Code to remove:

• createEndToEndScreenshotTest() in dev/devicelab/lib/tasks/integration_tests.dart