Remove temp index from members table

What does this MR do and why?

Removes the temp index tmp_index_members_on_state which was added in 14.7 and can now be removed.

The temporary index was added as part of !77298 (merged), although the migration file itself was removed/squashed in b7fbcab8.

Refs https://gitlab.com/gitlab-org/gitlab/-/issues/349960

Database info

migrate up

bin/rails db:migrate

main: == [advisory_lock_connection] object_id: 183000, pg_backend_pid: 75918
main: == 20231222114515 RemoveTmpIndexMembersOnState: migrating =====================
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0926s
main: -- indexes(:members)
main:    -> 0.0076s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0002s
main: -- remove_index(:members, {:algorithm=>:concurrently, :name=>"tmp_index_members_on_state"})
main:    -> 0.0034s
main: -- execute("RESET statement_timeout")
main:    -> 0.0005s
main: == 20231222114515 RemoveTmpIndexMembersOnState: migrated (0.1189s) ============

main: == [advisory_lock_connection] object_id: 183000, pg_backend_pid: 75918
ci: == [advisory_lock_connection] object_id: 183220, pg_backend_pid: 75921
ci: == 20231222114515 RemoveTmpIndexMembersOnState: migrating =====================
ci: -- transaction_open?(nil)
ci:    -> 0.0000s
ci: -- view_exists?(:postgres_partitions)
ci:    -> 0.0006s
ci: -- indexes(:members)
ci:    -> 0.0091s
ci: -- execute("SET statement_timeout TO 0")
ci:    -> 0.0005s
ci: -- remove_index(:members, {:algorithm=>:concurrently, :name=>"tmp_index_members_on_state"})
ci:    -> 0.0036s
ci: -- execute("RESET statement_timeout")
ci:    -> 0.0002s
ci: == 20231222114515 RemoveTmpIndexMembersOnState: migrated (0.0289s) ============

migrate down

bin/rails db:rollback:main
WARNING: This version of GitLab depends on gitlab-shell 14.32.0, but you're running 14.30.0. Please update gitlab-shell.
main: == [advisory_lock_connection] object_id: 182720, pg_backend_pid: 80499
main: == 20231222114515 RemoveTmpIndexMembersOnState: reverting =====================
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0848s
main: -- index_exists?(:members, :state, {:name=>"tmp_index_members_on_state", :where=>"state = 2", :algorithm=>:concurrently})
main:    -> 0.0071s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0001s
main: -- add_index(:members, :state, {:name=>"tmp_index_members_on_state", :where=>"state = 2", :algorithm=>:concurrently})
main:    -> 0.0043s
main: -- execute("RESET statement_timeout")
main:    -> 0.0002s
main: == 20231222114515 RemoveTmpIndexMembersOnState: reverted (0.1099s) ============

main: == [advisory_lock_connection] object_id: 182720, pg_backend_pid: 80499

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