Remove value `approvals` in `system_notes_metadata` table

We introduced system_note_metadata table in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1526 . We started to store approvals in action in case a MR was approved/unapproved.

This was changed in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1641 and we now store different values for approval and unapproval. However because the first change was already deployed in 9.1 we have some metadata with approvals as action.

We need to remove these data.