Remove `multiple_code_owner_rules` feature flag

What does this MR do?

Before we were creating a single ApprovalRule called "Code Owner". But when we introduced multiple_code_owner_rules we were creating a rule per entry in the CODEOWNERS file and displaying that.

Having the feature flag meant having to support the 2 datamodels. This removes the feature flag and gets rid of the legacy code that was there to support it's off-state.

Does this MR meet the acceptance criteria?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/9623

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content