Geo: sync disabled wikis. Stage 2

What does this MR do?

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.

CE port https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21903

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/6142 Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/7623

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?