Draft: Use route_setting authentication on runner get artifacts API

What does this MR do and why?

Related to #362336 (closed).

This is a follow up from !83713 (merged).

Since !83713 (merged) has been merged and the feature flags enabled, runners now use job token of the dependent job to request for job artifacts. The dependent job will always be running when the request is made.

With that in mind, we can remove the existing code that had to support either the token from the dependent job (still running) or from the artifact source job (completed job).

Since the requesting job is always running, we can use route_setting: :authentication, job_token_enabled: true which will ensure that the job is running.

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