Marks Mac_build_test flutter_gallery__transition_perf_e2e_ios to be unflaky (#138886)

<!-- meta-tags: To be used by the automation script only, DO NOT MODIFY.
{
  "name": "Mac_build_test flutter_gallery__transition_perf_e2e_ios"
}
-->
The test has been passing for [50 consecutive runs](https://data.corp.google.com/sites/flutter_infra_metrics_datasite/flutter_check_test_flakiness_status_dashboard/?p=BUILDER_NAME:%22Mac_build_test%20flutter_gallery__transition_perf_e2e_ios%22).
This test can be marked as unflaky.

.ci.yaml

@@ -4441,7 +4441,6 @@ targets:
   - name: Mac_build_test flutter_gallery__transition_perf_e2e_ios
     recipe: devicelab/devicelab_drone_build_test
     # TODO(vashworth): Take out of bringup once https://github.com/flutter/flutter/issues/138194 is resolved.
-    bringup: true
     timeout: 60
     properties:
       tags: >