Drop trace-inclusive artifact removal index from ci_job_artifacts

What does this MR do and why?

This MR drops a partial index we used to queue up artifacts that have expired and need to be removed from ci_job_artifacts.

We've recently built a new partial index that's entirely the same, but additionally exclude trace-type artifacts since those are not expirable, i.e. have no expire_at value. The indexes are both on, and ordered by, expire_at so this older index has a big bunch of a few hundred million job traces at one end of the index that we'll never need or touch.

By also restricting to WHERE expire_at IS NOT NULL, we exclude job traces and reduce the size of the index by about 86%:

Index sizes from db-lab:

Schema	Name	Type	Owner	Table	Size	Description
public	ci_job_artifacts_expire_at_unlocked_idx	index	joe_dcimino	ci_job_artifacts	6602 MB	CREATE INDEX ci_job_artifacts_expire_at_unlocked_idx ON ci_job_artifacts USING btree (expire_at) WHERE (locked = 0);
public	index_ci_job_artifacts_on_expire_at_for_removal	index	joe_dcimino	ci_job_artifacts	906 MB	CREATE INDEX index_ci_job_artifacts_on_expire_at_for_removal ON ci_job_artifacts USING btree (expire_at) WHERE ((locked = 0) AND (expire_at IS NOT NULL));

The new index was built over the weekend of April 2-3, and as of April 4th I've verified that the new index has been built, and the planner has wisely switched over to using it instead of the one we're dropping:

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