Drop unused index on merge_requests table

What does this MR do?

9d8ebcfc introduced a temporary index on merge_requests, which this MR drops again.

This came up during gitlab-com/gl-infra/production#2424 (comment 381214360), though it wasn't the actual cause (it just happened to surface the problem described in #229852 (comment 381380624)).

I also checked the index isn't being actively used on the GitLab.com primary and replicas. ✔

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content