Drop ci_job_artifacts_use_primary_to_authenticate feature flag

What does this MR do and why?

This drops the ci_job_artifacts_use_primary_to_authenticate feature so that that the /api/v4/jobs/:id/artifacts uses the primary database to authenticate CI job tokens. This feature flag was introduced in !155684 (merged).

This adds some load to the CI primary and reduces erroneous 401 errors due to replication lag. From our testing over the last week the endpoint doesn't appear to cause appreciable load to the system.

Relates to #466145 (closed)