Remove index_security_scans_on_pipeline_id concurrently

What does this MR do and why?

This merge request removes index_security_scans_on_pipeline_id concurrently.

The query to drop the removal took ~88 ms on postgres.ai console:

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/17910/commands/59630

We use a regular post-deployment migration and drop the index concurrently as the query is way below timing guidelines. (i.e. 20 minutes)

Resolves #403258 (closed).

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