Marks Mac_android hot_mode_dev_cycle__benchmark to be unflaky (flutte…

…r#138472)

<!-- meta-tags: To be used by the automation script only, DO NOT MODIFY.
{
  "name": "Mac_android hot_mode_dev_cycle__benchmark"
}
-->
The issue flutter#138072 has been closed, and 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_android%20hot_mode_dev_cycle__benchmark%22).
This test can be marked as unflaky.

.ci.yaml

@@ -3749,7 +3749,6 @@ targets:
       task_name: hello_world_android__compile
 
   - name: Mac_android hot_mode_dev_cycle__benchmark
-    bringup: true # Flaky https://github.com/flutter/flutter/issues/138072
     recipe: devicelab/devicelab_drone
     presubmit: false
     timeout: 60