Optimise deployments table by removing an unused index

What does this MR do and why?

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

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

The corresponding async-removal MR is here: !119270 (merged). I have confirmed that this index is no longer in gprd.

Issue: #402516 (closed)

Screenshots or screen recordings

DB Migration output:

$ bundle exec rails db:migrate

main: == [advisory_lock_connection] object_id: 275520, pg_backend_pid: 25555
main: == 20230502014227 DropPartialIndexDeploymentsForProjectIdAndTag: migrating ====
main: -- transaction_open?()
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.1276s
main: -- indexes(:deployments)
main:    -> 0.0136s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0004s
main: -- remove_index(:deployments, {:algorithm=>:concurrently, :name=>"partial_index_deployments_for_project_id_and_tag"})
main:    -> 0.0039s
main: -- execute("RESET statement_timeout")
main:    -> 0.0003s
main: == 20230502014227 DropPartialIndexDeploymentsForProjectIdAndTag: migrated (0.1717s) 

main: == [advisory_lock_connection] object_id: 275520, pg_backend_pid: 25555
ci: == [advisory_lock_connection] object_id: 275740, pg_backend_pid: 25558
ci: == 20230502014227 DropPartialIndexDeploymentsForProjectIdAndTag: migrating ====
ci: -- transaction_open?()
ci:    -> 0.0000s
ci: -- view_exists?(:postgres_partitions)
ci:    -> 0.0009s
ci: -- indexes(:deployments)
ci:    -> 0.0122s
ci: -- execute("SET statement_timeout TO 0")
ci:    -> 0.0004s
ci: -- remove_index(:deployments, {:algorithm=>:concurrently, :name=>"partial_index_deployments_for_project_id_and_tag"})
ci:    -> 0.0041s
ci: -- execute("RESET statement_timeout")
ci:    -> 0.0004s
ci: == 20230502014227 DropPartialIndexDeploymentsForProjectIdAndTag: migrated (0.0358s) 

ci: == [advisory_lock_connection] object_id: 275740, pg_backend_pid: 25558

How to set up and validate locally

N/A

MR acceptance checklist

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