Send Cache-Control headers by default on API /archive endpoint

What does this MR do?

Removes the api_repository_archive_cache_headers feature flag. This gitlab_com_derisk flag guarded the Cache-Control/ETag caching headers on the REST repository archive endpoint (GET /projects/278964/repository/archive).

The rollout is complete, so the flag and its conditional check are removed, making the caching behavior the default for every request. The flag YAML definition, the conditional flag check in the API endpoint, and the flag-disabled specs are removed.

References

• Rollout issue: #604292 (closed)
• Feature issue: #604279 (closed)
• Introduced by: !242792 (merged)