fix(manager): drop APP_CONFIG from migration enum resource types

BA-6873 removed cls.APP_CONFIG from EntityType._resource_types() in the
original common enum but missed the migration copy, breaking the
enum-sync tests. Mirror the removal so the copy matches the original.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: fregataa

src/ai/backend/manager/models/rbac_models/migration/enums.py

@@ -141,7 +141,6 @@ def _resource_types(cls) -> set[EntityType]:
             cls.SESSION,
             cls.ARTIFACT,
             cls.ARTIFACT_REGISTRY,
-            cls.APP_CONFIG,
             cls.NOTIFICATION_CHANNEL,
             cls.NOTIFICATION_RULE,
             cls.MODEL_DEPLOYMENT,