Drop index_deployments_on_environment_id_and_iid_and_project_id

What does this MR do and why?

This is part of an epic to optimize the deployments table by dropping unused indexes: &10185

MR that drops the index asynchronously in production: !125253 (merged)

Issue: #402509 (closed)

Screenshots or screen recordings

Migration output:

main: == [advisory_lock_connection] object_id: 223820, pg_backend_pid: 25964
main: == 20230712055956 DropIndexDeploymentsOnEnvironmentIdAndIidAndProjectId: migrating 
main: -- transaction_open?()
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0865s
main: -- indexes(:deployments)
main:    -> 0.0087s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0002s
main: -- remove_index(:deployments, {:algorithm=>:concurrently, :name=>"index_deployments_on_environment_id_and_iid_and_project_id"})
main:    -> 0.0038s
main: -- execute("RESET statement_timeout")
main:    -> 0.0002s
main: == 20230712055956 DropIndexDeploymentsOnEnvironmentIdAndIidAndProjectId: migrated (0.1210s) 

main: == [advisory_lock_connection] object_id: 223820, pg_backend_pid: 25964

Check that the index no longer exists on production DB

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 #402509 (closed)