In https://gitlab.com/gitlab-org/gitlab/-/merge_requests/77322 we
already added a ["loose foreign key"](
https://docs.gitlab.com/ee/development/database/loose_foreign_keys.html)
to replace this foreign key. Now that this has been running fine in
production we should be OK to remove the foreign key altogether.

This was added in a post-deployment migration as we want to ensure that
they have the correct code running with the loose foreign key in place
before we remove the foreign key to ensure that there isn't any lost
deletes.

Changelog: other