Marks Mac_ios draw_atlas_perf_ios__timeline_summary to be unflaky (#134626)

<!-- meta-tags: To be used by the automation script only, DO NOT MODIFY.
{
  "name": "Mac_ios draw_atlas_perf_ios__timeline_summary"
}
-->
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_ios%20draw_atlas_perf_ios__timeline_summary%22).
This test can be marked as unflaky.

.ci.yaml

@@ -2858,7 +2858,6 @@ targets:
     recipe: devicelab/devicelab_drone
     presubmit: false
     timeout: 60
-    bringup: true
     properties:
       tags: >
         ["devicelab", "ios", "mac"]