[macOS] Eliminate flutter_gallery_macos__start_up benchmark (#147442)

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 flutter/engine#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](https://github.com/flutter/flutter/blob/9d47055640a90879cfb45b2da55283504bedc354/dev/devicelab/lib/tasks/perf_tests.dart#L963)) 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.

.ci.yaml

@@ -3604,17 +3604,6 @@ targets:
         ]
       task_name: flutter_gallery_macos__compile
 
-  - name: Mac_benchmark flutter_gallery_macos__start_up
-    presubmit: false
-    recipe: devicelab/devicelab_drone
-    timeout: 60
-    properties:
-      dependencies: >-
-        [
-          {"dependency": "ruby", "version": "ruby_3.1-pod_1.13"}
-        ]
-      task_name: flutter_gallery_macos__start_up
-
   - name: Mac flutter_packaging_test
     recipe: packaging/packaging
     presubmit: false