macOS is missing many device lab tests that are running on iOS, Android, and Windows. Having flutter_gallery_mac__start_up and flutter_gallery_macos__start_up device lab tests is confusing.

Since we have other tests/benchmarks that can launch an Android app from a mac host, we can just delete flutter_gallery_mac__start_up. This will eliminate the confusion once flutter_gallery_macos__start_up is landed as a perf test for macOS.