Remove redundant scope `with_files_stored_locally`

What does this MR do and why?

It came up in an earlier MR that the scope with_files_stored_locally is actually redundant in MergeRequestDiff model. This MR removes it.

Screenshots or screen recordings

Testing locally:

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.