Draft: Remove background_upload code

Since we merged !97170 (merged), the background_upload config setting has been hard-coded to false. This MR prunes code and tests that only apply when background_upload is true, which is now unreachable anyway.

• Remove ObjectStorage::BackgroundMoveWorker because it is a no-op when background upload is disabled
• Remove ObjectStorage::BackgroundMove concern because its purpose is to schedule ObjectStorage::BackgroundMoveWorker
• Remove #background_upload_enabled? method from uploaders because it is always false now
• Remove #schedule_background_upload calls from uploaders because it is a no-op when background upload is disabled
• Remove configuration logic for the background_upload field
• Remove tests that test things that can no longer happen
• Remove documentation about how background upload works
• Mark Product Intelligence metrics about background_upload as "broken"

Part of gitlab-com/gl-infra/scalability#1690 (closed)