Object Storage: remove background upload

This is a placeholder for a project.

Outcome

1. For all object storage locations in the application, the application behaves as if background_upload is set to false
2. All code paths for background_upload: true are removed.

Related work

• Deprecate background upload in 15.0 (communicates intention to remove) gitlab-org/gitlab#26600 (closed)
• Document migration path for affected storage providers gitlab-org/gitlab!84084 (merged)