Mark animated_complex_opacity_perf_macos__e2e_summary unflaky (flutter#160996)

This should have been marked unflaky when
flutter#157423 was closed.

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

Author: jmagman

.ci.yaml

@@ -3708,7 +3708,6 @@ targets:
         ["framework","hostonly","shard","linux"]
 
   - name: Mac_benchmark animated_complex_opacity_perf_macos__e2e_summary
-    bringup: true # Flaky https://github.com/flutter/flutter/issues/157423
     presubmit: false
     recipe: devicelab/devicelab_drone
     timeout: 60