Drop index_deployments_on_id_where_cluster_id_present

What does this MR do and why?

This change is part of an epic to optimise the deployments table: &10185

This will drop the index_deployments_on_id_where_cluster_id_present index concurrently. It has a usage rate of 0 according to Thanos.

The corresponding async-removal MR is here: !129676 (merged). I have confirmed that this index is no longer in gprd.

Issue: Remove index_deployments_on_id_where_cluster_id... (#402510 - closed)

Screenshots or screen recordings

DB Migration output:

main: == [advisory_lock_connection] object_id: 226400, pg_backend_pid: 37231
main: == 20230904100544 DropIndexDeploymentsOnIdWhereClusterIdPresent: migrating ====
main: -- transaction_open?()
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.1052s
main: -- indexes(:deployments)
main:    -> 0.0104s
main: -- current_schema()
main:    -> 0.0003s
main: == 20230904100544 DropIndexDeploymentsOnIdWhereClusterIdPresent: migrated (0.1252s) 

main: == [advisory_lock_connection] object_id: 226400, pg_backend_pid: 37231

How to set up and validate locally

N/A

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 #402510 (closed)