Geo: sync disabled wikis. Stage 2

In https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6420 we took care of the smooth update and we kept repositories_count and wikis_count in place for backward compatibility when we update GitLab geo setup.

In this MR we remove these obsolete columns and some code related to backward compatibility

We also keep these columns in API for few releases, so there will be stage 3 yet.