Removes temporary index on project_statistics

What does this MR do and why?

Removes temporary index on project_statistics that was added in 031e79cb.

We previously added the temporary index on the updated_at column to support a background migration to backfill data.

The backfill completed in October '23, so we can now remove this index.

This will also assist with https://gitlab.com/gitlab-com/gitlab-OKRs/-/work_items/7861#note_1917578083

Refs #467342 (closed)

migrate up

 bin/rails db:migrate
main: == [advisory_lock_connection] object_id: 125720, pg_backend_pid: 83724
main: == 20240530114822 RemoveTmpIndexProjectStatisticsUpdatedAt: migrating =========
main: -- index_exists?(:project_statistics, [:project_id, :updated_at], {:name=>"tmp_index_project_statistics_updated_at"})
main:    -> 0.0326s
main: == 20240530114822 RemoveTmpIndexProjectStatisticsUpdatedAt: migrated (0.0472s) 

main: == [advisory_lock_connection] object_id: 125720, pg_backend_pid: 83724

migrate down

 bin/rails db:migrate:down:main VERSION=20240530114822
main: == [advisory_lock_connection] object_id: 125420, pg_backend_pid: 84236
main: == 20240530114822 RemoveTmpIndexProjectStatisticsUpdatedAt: reverting =========
main: == 20240530114822 RemoveTmpIndexProjectStatisticsUpdatedAt: reverted (0.0250s) 

main: == [advisory_lock_connection] object_id: 125420, pg_backend_pid: 84236

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