Remove packages_size index from project_statistics

What does this MR do and why?

Removes index_project_statistics_on_packages_size_and_project_id index from project_statistics.

Synchronously removes the index on project_statistics for packages size and project id, only for GitLab.com, which was removed async as part of #467342 (closed) in !156008 (merged).

Refs #466691 (closed)

Database

Migrate up:

bin/rails db:migrate
main: == [advisory_lock_connection] object_id: 128820, pg_backend_pid: 44710
main: == 20240708121845 RemoveIdxProjectStatisticsPackagesSizeAndProjectIdSync: migrating 
main: == 20240708121845 RemoveIdxProjectStatisticsPackagesSizeAndProjectIdSync: migrated (0.0049s) 

Migrate down:

bin/rails db:migrate:down:main VERSION=20240708121845
main: == [advisory_lock_connection] object_id: 128460, pg_backend_pid: 1076
main: == 20240708121845 RemoveIdxProjectStatisticsPackagesSizeAndProjectIdSync: reverting 
main: == 20240708121845 RemoveIdxProjectStatisticsPackagesSizeAndProjectIdSync: reverted (0.0043s) 

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