Delete repositories from unassigned storages
With #2971 (closed), Praefect has gained support for variable replication factor. Unassigned storage's may contain a copy of the repository if the assignments are changed after the repository has already been replicated there. Repositories won't be kept in-sync by Praefect on unassigned storage nodes. Praefect's reconciler should delete copies from unassigned storages to free up storage space where possible. Praefect should only do so given the repository's replication factor has been met. If the replication factor has not yet been met, it would be a good safety net to keep the copies around still. This is also necessary when moving a repository to another node, as it could be we are moving the only copy of the repository and need to keep it around until it has been successfully replicated by the new host storage.