Remove `index_identities_on_provider` DB index

What does this MR do and why?

We added index_identities_on_provider DB index to speed up

• db/post_migrate/20240419140530_set_trusted_extern_uid_to_false_for_existing_bitbucket_identities.rb in https://gitlab.com/gitlab-org/security/gitlab/-/merge_requests/3962
• db/post_migrate/20240603152046_delete_bitbucket_identities_with_untrusted_extern_uid.rb in !155094 (merged)

DB migrations. Since this index is not used by the application we should remove it.

Related to https://gitlab.com/gitlab-org/security/gitlab/-/merge_requests/3962/diffs#note_1874744321, https://gitlab.com/gitlab-org/gitlab/-/issues/456424

DB migration

bin/rails db:migrate RAILS_ENV=test

main: == [advisory_lock_connection] object_id: 128540, pg_backend_pid: 27934
main: == 20240607105207 RemoveIndexIdentitiesOnProvider: migrating ==================
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0053s
main: -- indexes(:identities)
main:    -> 0.0062s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0010s
main: -- remove_index(:identities, {:algorithm=>:concurrently, :name=>"index_identities_on_provider"})
main:    -> 0.0030s
main: -- execute("RESET statement_timeout")
main:    -> 0.0012s
main: == 20240607105207 RemoveIndexIdentitiesOnProvider: migrated (0.0419s) =========

main: == [advisory_lock_connection] object_id: 128540, pg_backend_pid: 27934
ci: == [advisory_lock_connection] object_id: 128720, pg_backend_pid: 27936
ci: == 20240607105207 RemoveIndexIdentitiesOnProvider: migrating ==================
ci: -- transaction_open?(nil)
ci:    -> 0.0000s
ci: -- view_exists?(:postgres_partitions)
ci:    -> 0.0012s
ci: -- indexes(:identities)
ci:    -> 0.0064s
ci: -- execute("SET statement_timeout TO 0")
ci:    -> 0.0011s
ci: -- remove_index(:identities, {:algorithm=>:concurrently, :name=>"index_identities_on_provider"})
ci:    -> 0.0023s
ci: -- execute("RESET statement_timeout")
ci:    -> 0.0015s
ci: == 20240607105207 RemoveIndexIdentitiesOnProvider: migrated (0.0412s) =========

ci: == [advisory_lock_connection] object_id: 128720, pg_backend_pid: 27936

bin/rails db:migrate:down:main VERSION=20240607105207 RAILS_ENV=test

main: == [advisory_lock_connection] object_id: 117240, pg_backend_pid: 26723
main: == 20240607105207 RemoveIndexIdentitiesOnProvider: reverting ==================
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0031s
main: -- index_exists?(:identities, :provider, {:name=>"index_identities_on_provider", :algorithm=>:concurrently})
main:    -> 0.0033s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0005s
main: -- add_index(:identities, :provider, {:name=>"index_identities_on_provider", :algorithm=>:concurrently})
main:    -> 0.0018s
main: -- execute("RESET statement_timeout")
main:    -> 0.0005s
main: == 20240607105207 RemoveIndexIdentitiesOnProvider: reverted (0.0244s) =========

main: == [advisory_lock_connection] object_id: 117240, pg_backend_pid: 26723

bin/rails db:migrate:down:ci VERSION=20240607105207 RAILS_ENV=test

ci: == [advisory_lock_connection] object_id: 117240, pg_backend_pid: 27103
ci: == 20240607105207 RemoveIndexIdentitiesOnProvider: reverting ==================
ci: -- transaction_open?(nil)
ci:    -> 0.0000s
ci: -- view_exists?(:postgres_partitions)
ci:    -> 0.0030s
ci: -- index_exists?(:identities, :provider, {:name=>"index_identities_on_provider", :algorithm=>:concurrently})
ci:    -> 0.0032s
ci: -- execute("SET statement_timeout TO 0")
ci:    -> 0.0004s
ci: -- add_index(:identities, :provider, {:name=>"index_identities_on_provider", :algorithm=>:concurrently})
ci:    -> 0.0021s
ci: -- execute("RESET statement_timeout")
ci:    -> 0.0005s
ci: == 20240607105207 RemoveIndexIdentitiesOnProvider: reverted (0.0304s) =========

ci: == [advisory_lock_connection] object_id: 117240, pg_backend_pid: 27103

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