Cleanup the remaining parts of `approvals_before_merge` field usage
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Must be done after #353097.
After `approvals_before_merge` is removed within https://gitlab.com/groups/gitlab-org/-/epics/7572, we should clean up the code that has been using `approvals_before_merge` column. It includes removal of [DeprecatedApprovalsBeforeMerge](https://gitlab.com/gitlab-org/gitlab/blob/0b62424123c2b721a9cad8608cda4933ae5ee780/ee/app/models/concerns/deprecated_approvals_before_merge.rb#L4).
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