Synchronous removal of project statistics tmp_index_project_statistics_updated_at

Summary

We previously added the temporary index on the updated_at column to support a background migration to backfill data.

The backfill completed in October '23, so we can now remove this index.

This will also assist with https://gitlab.com/gitlab-com/gitlab-OKRs/-/work_items/7861#note_1917578083

In !154410 (merged) we are asynchronously removing the index due to it being on a large table.

The purpose of this issue is to handle the synchronous removal of the index once that has completed