Clean up unused metadata fetchers and metadata authorities

If we remove enough RawExtrinsicMetadata, we might reach a point where a MetdataAuthority or MetadaFetcher gets unreferenced. They should probably be cleaned up if that’s the case.

If we ever remove MetadataAuthority or MetadataFetcher objects, extra care must be taken when restoring a recovery bundle: we need to be sure that they exists in the storage before restoring any RawExtrinsicMetadata using them. Maybe that means we need to backup them in every bundles?