Remove duplicated index from merge_request_assignees

What does this MR do and why?

Remove index_merge_request_assignees_on_merge_request_id duplicated index from merge_request_assignees.

This index is already covered by index_merge_request_assignees_on_merge_request_id_and_user_id

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.

Related to #428560 (closed)