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

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) 

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.