Remove NamespaceService
Prior to hashed storage, group's projects were stored under a directory path named after the group. The NamespaceService was used for operations covering these directories, including creating, renaming and checking for their existence. The service's RPC haven't been used since, as can be seen in the dashboards. We should remove the service as it is unused. Prior to removing them, we should make sure to remove any code and tests related to them from Rails so the pipeline won't fail.