Marks Mac_ios very_long_picture_scrolling_perf_ios__e2e_summary to be unflaky (#134629)

<!-- meta-tags: To be used by the automation script only, DO NOT MODIFY.
{
  "name": "Mac_ios very_long_picture_scrolling_perf_ios__e2e_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%20very_long_picture_scrolling_perf_ios__e2e_summary%22).
This test can be marked as unflaky.

.ci.yaml

@@ -4313,7 +4313,6 @@ targets:
 
   - name: Mac_ios very_long_picture_scrolling_perf_ios__e2e_summary
     recipe: devicelab/devicelab_drone
-    bringup: true
     presubmit: false
     timeout: 120
     properties: