Unlink Git LFS designs after they have been deleted

This issue came out of this discussion #38048 (comment 331919259).

When designs are deleted, we currently keep them around in Git LFS. We could unlink the LfsObjectsProject record where repository_type was design. If there were no other Git LFS links to the object, it might allow RemoveUnreferencedLfsObjectsWorker to do the destroying when it next ran.