Resolve "`Remove source branch` button sometimes appears even if `remove branch` checkbox is checked"

What does this MR do?

• Ensure Remove branch button doesn't show when MR has been set to be deleted on merge

Why was this MR needed?

• Remove branch was appearing even when checkbox was selected when Editing / Creating MR

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #28772 (closed)