Clean up bitbucket_server_notes_separate_worker

This issue is to remove the remaining code left after the bitbucket_server_notes_separate_worker feature flag removal.

The remaining code was left in place to avoid issues during in-flight migrations on self-managed instances that may have manually disabled the feature flag, so they still use the previous method to import notes.

To reduce the risk of migrations to fail, this issue should be completed one release after the feature flag removal #456262 (closed)