Remove deprecated code_owner attribute from approval_merge_request_rules
With the work in #9928 (closed) we migrated the boolean
approval_merge_request_rules.code_owner attribute to the
rule_type enum. Now that this migration has taken place with the release of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13036 in %12.0 we can drop the old column in %12.1
approval_merge_request_rules.code_owner column in favor of
All existing logic has been migrated to
rule_type however any missing conditions could lead to a failure to adequately display or update
code_owners on approval merge request rules.