Merge branch...

Merge branch '513174-feature-flag-rollout-of-show_diff_if_head_sha_commit_is_missing' into 'master' 

Remove show_diff_if_head_sha_commit_is_missing feature flag

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/182473



Merged-by: Andrew Evans <aevans@gitlab.com>
Approved-by: Dillon Wheeler <dwheeler@gitlab.com>
Approved-by: Andrew Evans <aevans@gitlab.com>
Reviewed-by: Gary Holtz <gholtz@gitlab.com>
Reviewed-by: Dillon Wheeler <dwheeler@gitlab.com>
Co-authored-by: Gary Holtz <gholtz@gitlab.com>