Merge branch 'drop-idx_merge_requests_on_target_project_id_and_iid_opened' into 'master'

Drop a partial index from merge_requests we no longer need

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/149218



Merged-by: Dylan Griffith <dyl.griffith@gmail.com>
Approved-by: Dylan Griffith <dyl.griffith@gmail.com>
Co-authored-by: David Kim <dkim@gitlab.com>