Revert "Re-enable Impeller goldens blocking. (#144023)" (#144115)

On the current head some tests (such as test/widgets/shadow_test.dart) are segfaulting in the Impeller Vulkan back end.

This reverts commit 39585e66.

See https://github.com/flutter/flutter/issues/144116

.ci.yaml

@@ -3458,6 +3458,7 @@ targets:
   - name: Mac framework_tests_impeller
     recipe: flutter/flutter_drone
     timeout: 60
+    bringup: true
     properties:
       cpu: x86 # https://github.com/flutter/flutter/issues/119880
       dependencies: >-