Resolve "remove source branch" checkbox from merge widget being ignored

What does this MR do?

This will fix #34897 (closed) making the merge widget "Remove source branch" being ignored.

Why was this MR needed?

The checkbox was ignored and the fallback was to use the "Remove source branch" in the MR creation settings. This caused the MR widget checkbox to be useless.

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

What are the relevant issue numbers?

Closes #34897 (closed)