Since https://gitlab.com/gitlab-org/gitlab/-/merge_requests/97170, 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 commit removes ObjectStorage::BackgroundMoveWorker. Part of
https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/1690.

Changelog: other