Drop index_projects_on_mirror_last_successful_update_at

What does this MR do and why?

Part of LW Locks: Investigate projects indexes that can... (&11760 - closed) and Reduce LockManager LWLock Contention (&11543 - closed).

Drops the index index_projects_on_mirror_last_successful_update_at on projects, which is unused on gitlab.com.

Usage report from Thanos: source

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #428834 (closed)