Remove FF ci_use_job_artifacts_table_for_exposed_artifacts

What does this MR do and why?

The feature flag ci_use_job_artifacts_table_for_exposed_artifacts was globally rolled out at 2025-08-19 1:38 PM PT and is deemed stable.

This feature was introduced in !199169 (merged) and does the following:

• If a job has exposed artifacts, the values exposed_as and exposed_paths will be persisted to the table p_ci_job_artifacts upon creating a new metadata artifact.
• When finding exposed artifacts to display in the MR UI, the service will first attempt to read from p_ci_job_artifacts if the data exists, otherwise it defaults to reading from p_ci_builds_metadata as it does currently.

References

• Roll out issue: [FF] `ci_use_job_artifacts_table_for_exposed_ar... (#557295 - closed)

How to set up and validate locally

See validation steps at !199169 (merged).

MR acceptance checklist

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 #557295 (closed)