This foreign key caused the incident
https://gitlab.com/gitlab-com/gl-infra/production/-/issues/19474 . It
was added in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/177799 . The
problem is that we added a foreign key before adding an index. This
means that all statements like `DELETE FROM projects ...` will need to
scan the entire `merge_request_diff_files` table to check if the foreign
key is valid. This was causing timeouts.

In order to resolve the incident we've already manually deleted this
migration on GitLab.com . This MR additionally adds another migration to
remove the foreign key to clean up any environments where this migration
may have already run.