Synchronous removal of `merge_requests_state_id_temp_index`

What does this MR do and why?

Synchronous removal of merge_requests_state_id_temp_index, which resolves `merge_requests_state_id_temp_index` synchronou... (#408766 - closed)

The async removal is here: Asynchronously remove `merge_request_state_id_t... (!120348 - merged)

How to set up and validate locally

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

1. Run rails db:migrate
2. Confirm merge_requests_state_id_temp_index no longer exists

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 #408766 (closed)

Related to #402497 (closed)