Draft: Remove code from problematic migration and restore structure.sql

What does this MR do and why?

This MR is part of a larger plan to mitigate a migration which is taking too long and blocking deployments.

This is the plan.

PLEASE REVIEW THIS PLAN (in addition to the source code change included by this MR)

Via @drew from slack:

What has already happened:

• We merged !84308 (merged) recently, during 14.10
• We dropped the index on staging
• We began the migration on production
• A production incident has been opened because the migration has been running for over an hour

What we would like to do:

1. [For @drew] Create the MR to revert the migration out of the release.
   • Done: !84987 (closed)
2. [For EOC (@nnelson)] Manually kill the migration on production to end the transaction.
   1. Create a production change request for this.
      • Done: gitlab-com/gl-infra/production#6822 (closed)
   2. Kill the deployment job.
   3. Check the status of the 82178 statement: DROP INDEX CONCURRENTLY "ci_job_artifacts_expire_at_unlocked_idx"
   4. If the process is still running, then forcibly cancel it.
      • https://ops.gitlab.net/gitlab-com/runbooks/-/blob/master/docs/patroni/postgresql-locking.md#how-to-cancel-or-terminate-a-blocking-query
   5. If the process is still running, then forcible terminate it.
3. Find a reviewer+merger (backend maintainer) to merge the new MR from step 1..
4. Merge the new MR from step 1. that removes the actual migration code from #up and #down in the existing, previously merged migration effectively making it a no-op
5. [For @rpereira2] Validate that the auto-deploy package contains the commit from the revert MR.
6. [Anyone] Manually run migrate:down on staging only, to rebuild the index and bring the staging database back into parity with the production database
   1. Shell into the console-01-sv-gstg.c.gitlab-staging-1.internal system.
   2. Invoke: rake db:migrate:down VERSION=20220404184814
7. Deploy again to staging and production, which will both run an empty migration, successfully, and leave the indexes in place in both environments

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