What does this MR do and why?

This MR removes unused userSelect from app/assets/javascripts/pages/projects/merge_requests/index/index.js.

Screenshots or screen recordings

No visual changes.

How to set up and validate locally

1. Go to the merge requests page.
2. Click Edit and select some MRs.
3. Verify the assignee select works as expected.

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.

Review progress:

• frontend developer review
• frontend maintainer review

Related to #369997 (closed)