Remove feature flag locked_lfs_files_mergeability_check

Gavin Hinfeyrequested to merge
478162-cleanup-locked_lfs_files_mergeability_check into master

What does this MR do and why?

Removes the locked_lfs_files_mergeability_check feature flag #478162 (closed). Roll out issue: #474485 (closed).

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Edited by Gavin Hinfey

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