Drop stale temporary index on ci_job_artifacts

What does this MR do and why?

This migration drops a temporary index that we're not using anymore. The schema change was introduced in !79547 (merged), after the index was originally built asynchronously over the weekend.

A background migration ran to clear all the timestamps from the targeted trace artifacts, and we've since verified that there are none left:

This index should be empty, which we can be checked as part of the database review.

Closes #352772 (closed)

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