Marks Linux_pixel_7pro picture_cache_perf__timeline_summary to be unflaky (#134118)

<!-- meta-tags: To be used by the automation script only, DO NOT MODIFY.
{
  "name": "Linux_pixel_7pro picture_cache_perf__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:%22Linux_pixel_7pro%20picture_cache_perf__timeline_summary%22).
This test can be marked as unflaky.

.ci.yaml

@@ -2493,7 +2493,6 @@ targets:
 
   - name: Linux_pixel_7pro picture_cache_perf__timeline_summary
     recipe: devicelab/devicelab_drone
-    bringup: true
     presubmit: false
     timeout: 60
     properties: