Remove feature flag for named `multiple_code_owner_rules`
When removing the
multiple_code_owner_rules (introduced in !9455 (merged)) and
code_owner_rules feature flag, we can remove a bunch of other legacy code as well.
EE::MergeRequests#sync_code_owners_with_approversin favor of the
ApprovalMergeRequestRule::DEFAULT_NAME_FOR_CODE_OWNERcan be removed, the rules are created with their pattern instead of a default name
EE::MergeRequest#code_ownerscan be removed this is relaced by separate entries displayed on the merge request widget through approval rules
Gitlab::CodeOwners.for_merge_requestcan be removed. We should only be loading entries, which map a pattern to a set of users.
EE::MergeRequest::RefreshService#update_approverssupport for old approver version can be removed
EE::MergeRequests::RefreshService#create_approverswhich is relaced with the new approval rules model