Remove codeowners_match_ancestor_groups feature flag

What does this MR do?

Removes the codeowners_match_ancestor_groups flag. We added this functionality in c099a291 as part of addressing the bug https://gitlab.com/gitlab-org/gitlab/-/issues/216345 as well as the long-requested feature #32432 (closed) but sinec we had experienced a fair amount of bugs in that feature, we used a FF to be able to circuit break if the behavior was incorrect.

(Note that tests were removed in a previous commit ~4 releases ago)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team