Remove ObjectStorage::BackgroundMoveWorker

Since !97170 (merged), the object storage background_upload setting is hard coded to false. This makes ObjectStorage::BackgroundMoveWorker a no-op and prevents jobs for this worker to be scheduled in the first place.

This MR removes ObjectStorage::BackgroundMoveWorker. We intentionally do not remove more code because that would make the MR too big. There will be follow-up MR's, tracked in gitlab-com/gl-infra/scalability#1690 (closed), for removing the rest of the background upload code.