Stop validating vanity repo exists in BackupRepository

The vanity repository is used to generate paths to store backups since we want to store backups using the relative path that gitlab-rails knows, not the rewritten replica path. As such the vanity repository is only expected to exist in praefect so there is no way for gitaly to properly validate this other than simply checking that the fields are set.

RestoreRepository already handles this correctly. Unfortunately this RPC was created between the repository validation unification and the validates storage bug fix.