chore(seer grouping): Remove projects:similarity-embeddings-grouping feature flag (#77123)

This is a follow-up to #77116 and https://github.com/getsentry/getsentry/pull/15056, which remove all usage of, and the feature handler for, the `projects:similarity-embeddings-grouping` feature flag (respectively), removing the flag itself.

Author: lobsterkatie

src/sentry/features/temporary.py

@@ -543,8 +543,6 @@ def register_temporary_features(manager: FeatureManager):
     manager.add("projects:similarity-embeddings", ProjectFeature, FeatureHandlerStrategy.INTERNAL, default=False, api_expose=True)
     manager.add("projects:similarity-embeddings-backfill", ProjectFeature, FeatureHandlerStrategy.OPTIONS, api_expose=False)
     manager.add("projects:similarity-embeddings-delete-by-hash", ProjectFeature, FeatureHandlerStrategy.OPTIONS, api_expose=False)
-    # Enable using Seer to group events
-    manager.add("projects:similarity-embeddings-grouping", ProjectFeature, FeatureHandlerStrategy.INTERNAL, default=False, api_expose=False)
     manager.add("projects:similarity-indexing", ProjectFeature, FeatureHandlerStrategy.INTERNAL, api_expose=False)
     manager.add("projects:similarity-view", ProjectFeature, FeatureHandlerStrategy.INTERNAL, api_expose=True)
     # Starfish: extract metrics from the spans