Asynchronously remove `merge_request_state_id_temp_index` (again)

What does this MR do and why?

From Remove unused indexes on `merge_requests` table (#402497 - closed), this index is no longer used and can be removed.

Unfortunately, the previous attempt of this (Async remove 'merge_requests_state_id_temp_inde... (!116729 - merged)) did not remove the index because the index should be on id and not state_id (despite what the name says).

This attempt should take care of it 😄

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Related to #408766 (closed)