Remove codeowner_role_approvers ff

What does this MR do and why?

Cleans up the codeowner_role_approvers feature flag which was set to default enabled in 17.8.

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

Feature: Ability to reference Maintainers or Developers ... (#282438 - closed)
Rollout issue: [Feature flag] Rollout of `codeowner_role_appro... (#497504 - closed)
Cleanup issue: [Feature flag] Cleanup codeowner_role_approvers (#512623 - closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #512623 (closed)