Remove `repository_storage` from settings API
https://gitlab.com/gitlab-org/gitlab-ee/blob/v8.17.0-ee/lib/api/settings.rb#L121-L122
We used to have a single field for this - now we have an array, to pick one at random. This is the only place that repository_storage should be removed; on a project level, a project should still have a single storage location.