[macOS] Eliminate flutter_gallery_macos__start_up benchmark

[cbracken]

The macOS devicelab machines are not physically configured for benchmarking at a 60 Hz refresh rate. With Flutter vsync wired up via CVDisplayLink (rather than a hardcoded 60 Hz rate as previously), the bot shows 0 monitors configured and syncs at 30 Hz. This is not a meaningful target to measure against but consumes devicelab CPU cycles.

Given that there is currently no plan in place to acquire and configure a consistent pool of physical benchmarking desktop machines, disabling this test to reduce load on the bots.

See https://github.com/flutter/engine/pull/51210 for a discussion of the decision to move forward with wiring up vsync from macOS and ignore this benchmark.

For future archaeologists, I don't think it's worth removing all the plumbing in the Dart code (in dev/devicelab/lib/tasks/perf_tests.dart) that supports running this benchmark on macOS. If we ever acquire hardware suitable for benchmarking with the appropriate HDMI dongle to simulate a 60Hz 4k display, for example, it would be reasonable to re-enable this benchmark.

Pre-launch Checklist

• [X] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [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.
• [X] I listed at least one issue that this PR fixes in the description above.
• [X] I updated/added relevant documentation (doc comments with ///).
• [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.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[auto-submit[bot]]

auto label is removed for flutter/flutter/147442, due to - The status or check suite Google testing has failed. Please fix the issues identified (or deflake) before re-applying this label.