[Feature flag] Cleanup search_mr_filter_source_branch

What does this MR do and why?

Removed feature flag search_mr_filter_source_branch

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• #508634 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After

How to set up and validate locally

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

1. Make sure advanced search is installed and enabled
2. Visit search page eg. http://gdk.test:3000/search?scope=merge_requests&search=*&group_id=35
3. Search anything in merge request scope and select group or project