Cleanup 'locked_paths_mergeability_check' FF

Ash McKenzierequested to merge
489848-feature-flag-cleanup-locked_paths_mergeability_check into master

What does this MR do and why?

This MR removes all references of the locked_paths_mergeability_check feature flag.

Related: [Feature Flag] Cleanup `locked_paths_mergeabil... (#489848 - closed)

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