Remove bringup: true from web test shard. (#150785)

These two fixes seem to have stopped the flaking:
https://github.com/flutter/flutter/pull/149828
https://dart-review.googlesource.com/c/sdk/+/367821

.ci.yaml

@@ -1294,7 +1294,6 @@ targets:
 
   - name: Linux web_long_running_tests_2_5
     recipe: flutter/flutter_drone
-    bringup: true
     timeout: 60
     properties:
       dependencies: >-