Remove feature flag for named `multiple_code_owner_rules`

When removing the multiple_code_owner_rules (introduced in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9455) and code_owner_rules feature flag, we can remove a bunch of other legacy code as well.

• EE::MergeRequests#sync_code_owners_with_approvers in favor of the MergeRequests::SyncCodeOwnerApprovalRulesservice
• ApprovalMergeRequestRule::DEFAULT_NAME_FOR_CODE_OWNER can be removed, the rules are created with their pattern instead of a default name
• EE::MergeRequest#code_owners can be removed this is relaced by separate entries displayed on the merge request widget through approval rules
• Gitlab::CodeOwners.for_merge_request can be removed. We should only be loading entries, which map a pattern to a set of users.
• EE::MergeRequest::RefreshService#update_approvers support for old approver version can be removed
• EE::MergeRequests::RefreshService#create_approvers which is relaced with the new approval rules model