Remove unused index index_ci_job_artifacts_on_expire_at_for_removal synchronously

What does this MR do and why?

#393913 (closed)

Drop unused index index_ci_job_artifacts_on_expire_at_for_removal synchronously.

Index is no longer used since it was replaced by index_ci_job_artifacts_expire_at_unlocked_non_trace.

https://thanos.gitlab.net/graph?g0.expr=sum(rate(pg_stat_user_indexes_idx_scan%7Benv%3D%22gprd%22%2C%20indexrelname%3D%22index_ci_job_artifacts_on_expire_at_for_removal%22%2C%20type%3D%22patroni-ci%22%7D%5B5m%5D))&g0.tab=0&g0.stacked=0&g0.range_input=8w&g0.max_source_resolution=0s&g0.deduplicate=1&g0.partial_response=0&g0.store_matches=%5B%5D&g1.expr=sum(rate(pg_stat_user_indexes_idx_scan%7Benv%3D%22gprd%22%2C%20indexrelname%3D%22index_ci_job_artifacts_expire_at_unlocked_non_trace%22%2C%20type%3D%22patroni-ci%22%7D%5B5m%5D))&g1.tab=0&g1.stacked=0&g1.range_input=8w&g1.max_source_resolution=0s&g1.deduplicate=1&g1.partial_response=0&g1.store_matches=%5B%5D

This MR should only be merged after the async removal has been merged and the index has been dropped on GitLab.com.

✅ the index has been dropped on GitLab.com: !116642 (comment 1346345281)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #393913 (closed)