Drop the `merge_train_to_canonical` FF from the release-tools code

The merge_train_to_canonical FF was introduced as part of #19402 (closed) as a way to allow an independent sync for the GitLab repository via merge train. For context, the FF is also used as a safeguard if merge-train is not able to successfully sync changes, so the FF should be removed once there is an alternative to sync the changes without the merge train.