Schedule the removal of index_merge_request_diffs_on_project_id

What does this MR do and why?

Schedules the removal of a now redundant index index_merge_request_diffs_on_project_id. This index is being replaced by index_merge_request_diffs_on_project_id_and_id added in index_merge_request_diffs_on_project_id_and_id ... (#520027 - closed)

References

• index_merge_request_diffs_on_project_id databas... (#521444 - closed) - Async removal
• index_merge_request_diffs_on_project_id synchro... (#526146 - closed) - Sync removal followup

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