Resolve "NoMethodError: undefined method `uploader_context' for #<ObjectStorage::MigrateUploadsWorker::Upload:0x00007f59e..."

What does this MR do?

Remove the model redefinition in the MigrateUploadsWorker which caused a problem when moving FileUploader-based uploads.

Are there points in the code the reviewer needs to double check?

The redefinition was there because the worker acts as a background job, and I wanted to ensure the resilence of the model. In fact, I think that is a wrong assumption and we should rather ensure the tests are not broken in the future.

Why was this MR needed?

See #46147 (closed)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #46147 (closed)

/cc @ayufan @ahanselka @jprovaznik