Remove code_review duplicated indexes

After #385701 (closed), we can identify duplicated indexes. At the moment, there are many duplicate indexes that can be removed from our database, as they are not needed anymore.

The list of duplicated indexes can be found in the duplicate_indexes.yml file.

We have identified the following duplicated indexes owned by groupcode review:

merge_request_assignees table

• index_merge_request_assignees_on_merge_request_id_and_user_id: index_merge_request_assignees_on_merge_request_id

merge_requests table

• index_merge_requests_on_author_id_and_created_at: index_merge_requests_on_author_id
• index_merge_requests_on_author_id_and_id: index_merge_requests_on_author_id
• index_merge_requests_on_author_id_and_target_project_id: index_merge_requests_on_author_id

NOTE: Consider removing indexes showed on the right. Indexes at the left are covering for the duplicated one.

Feel free to split this issue into several under &11436 (closed) if that aligns more with how your group works.

See the references below to remove indexes. If the table is larger or with a lot of traffic, please consider dropping the index asynchronously.

References:

• https://docs.gitlab.com/ee/development/database/adding_database_indexes.html#drop-indexes-asynchronously
• https://docs.gitlab.com/ee/development/migration_style_guide.html#removing-indexes

Related #423983 (closed)

cc @alexives, @rogerwoo