Move drop_site_statistics to be a post deployment migration

What does this MR do?

This is a fix/follow up for the MR https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23314

By moving the migration to post deployment migration, we will not have to require a downtime.

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ce/issues/53778
• gitlab-org/release/tasks#568 (closed)