Mark complex_layout_scroll_perf_macos__timeline_summary unflaky (#160997)

This should have been marked unflaky when
https://github.com/flutter/flutter/issues/158251 was closed.

Co-authored-by: Chris Bracken <chris@bracken.jp>

.ci.yaml

@@ -3929,7 +3929,6 @@ targets:
       task_name: complex_layout_macos__start_up
 
   - name: Mac_benchmark complex_layout_scroll_perf_macos__timeline_summary
-    bringup: true # Flaky https://github.com/flutter/flutter/issues/158251
     presubmit: false
     recipe: devicelab/devicelab_drone
     timeout: 60