Remove feature gates for Repository Languages

What does this MR do?

Remove the feature flags around the Repository languages feature.

What are the relevant issue numbers?

Acceptance testing wasn't done through an issue, like the case is for Gitaly. So not really, in the future maybe through: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21156

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• Documentation created/updated
• [-] Tests added for this feature/bug
• [-] Conforms to the code review guidelines
• [-] Conforms to the merge request performance guidelines
• [-] Conforms to the style guides
• [-] Conforms to the database guides

Related issue

Closes #50291 (closed)