Synchronously remove unused index_merge_request_diffs_on_project_id

What does this MR do and why?

Synchronously remove unused index_merge_request_diffs_on_project_id

This MR is as per our DB guidelines https://docs.gitlab.com/development/database/adding_database_indexes/#add-a-migration-to-destroy-the-index-synchronously

References

• Issue Schedule the removal of index_merge_request_dif... (!185341 - merged)
• Schedule async removal MR Schedule the removal of index_merge_request_dif... (!185341 - merged) - %17.11

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