Remove legacy non-FDW queries from Geo

Over the past 2-3 releases we've added FDW-using alternatives to various queries that span the secondary's read-only database and the Geo tracking database.

In %10.3 we should be in a position where FDW can be configured automatically in omnibus and GDK installations. How to set it up for from-source installations should be documented.

What blockers remain to removing the non-FDW versions of the relevant queries? (mostly contained in https://gitlab.com/gitlab-org/gitlab-ee/blob/master/ee/app/finders/geo)

• We currently fall back to legacy queries when using selective sync. (Issue to resolve this https://gitlab.com/gitlab-org/gitlab-ee/issues/8798)
• We currently fall back to legacy queries for COUNT operations when using pg < 10.0

Once everything above is resolved, we can make FDW mandatory and remove the legacy queries. Having to make changes in two places slows us down.

/cc @jramsay @brodock