Remove possibly unused index on events table

The following discussion from !17890 (merged) should be addressed:

• @yorickpeterse started a discussion: (+3 comments)

  If we flip this index around so it's (author_id, project_id) then I think we can drop the standalone index on author_id.

-> Check if this index is actually still used after the changes from !17890 (merged) have been deployed.

"index_events_on_author_id" btree (author_id)