Removes repository_size index on project_statistics async

What does this MR do and why?

In this merge request, we are removing the index_project_statistics_on_repository_size_and_project_id index for GitLab.com.

The index_project_statistics_on_repository_size_and_project_id is only used in self-host users. We don't need this index in GitLab.com.

More info here: https://gitlab.com/gitlab-com/gitlab-OKRs/-/work_items/7861#note_1926540410

Refs #467342 (closed)

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