Removes tmp_index_project_statistics_updated_at from project statistics

What does this MR do and why?

Removes tmp_index_project_statistics_updated_at from project statistics.

It is a temporary index that was removed async in !154410 (diffs) last week

db:migrate up

main: == [advisory_lock_connection] object_id: 88780, pg_backend_pid: 77581
main: == 20240611103909 RemoveTmpIndexProjectStatisticsUpdatedAtSync: migrating =====
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0131s
main: -- indexes(:project_statistics)
main:    -> 0.0079s
main: -- current_schema(nil)
main:    -> 0.0002s
main: == 20240611103909 RemoveTmpIndexProjectStatisticsUpdatedAtSync: migrated (0.0437s) 

main: == [advisory_lock_connection] object_id: 88780, pg_backend_pid: 77581

db:migrate down

 bin/rails db:migrate:down:main VERSION=20240611103909
main: == [advisory_lock_connection] object_id: 88320, pg_backend_pid: 84570
main: == 20240611103909 RemoveTmpIndexProjectStatisticsUpdatedAtSync: reverting =====
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0119s
main: -- index_exists?(:project_statistics, [:project_id, :updated_at], {:name=>"tmp_index_project_statistics_updated_at", :where=>"repository_size > 0", :algorithm=>:concurrently})
main:    -> 0.0114s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0002s
main: -- add_index(:project_statistics, [:project_id, :updated_at], {:name=>"tmp_index_project_statistics_updated_at", :where=>"repository_size > 0", :algorithm=>:concurrently})
main:    -> 0.0111s
main: -- execute("RESET statement_timeout")
main:    -> 0.0002s
main: == 20240611103909 RemoveTmpIndexProjectStatisticsUpdatedAtSync: reverted (0.0681s) 

Refs #464566

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