Remove bringup:true from Linux tool_tests_widget_preview_scaffold (flutter#166687)

New shard seems to be stable and passing consistently.

Author: bkonyi

.ci.yaml

@@ -1592,7 +1592,6 @@ targets:
   - name: Linux tool_tests_widget_preview_scaffold
     recipe: flutter/flutter_drone
     timeout: 60
-    bringup: true
     properties:
       add_recipes_cq: "true"
       dependencies: >-