Index Removal: index_ci_job_artifacts_on_expire_at_and_job_id

The Problem

The index_ci_job_artifacts_on_expire_at_and_job_id index was identified as potentially unused, and one of the top three that represent over 60% of our potentially wasted index space.

A Proposal

Drop the index. Before we do this, we need to ensure that the index is in fact unused by any current workflows. It's possible for indexes to appear they have no traffic because they are only used sporadically for large operations, such as migrations. We need to be careful about this.