Remove deprecated `code_owner` references from ApprovalMergeRequestRule

As part of #11834 (closed), we want to code that refers to a soon-to-be ignored/removed column code_owner. This MR cleans up technical debt over a year old, and streamlines us to use existing ActiveRecord::Enum methods, as we can rely on the rule_type column going forward (as we have for a year or so now..)