Dry up mentions of NTP-compatible

Follow up from Sid's demo;

Sid: Why do we have 3 mentions of 'NTP-compatible' in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3485/diffs ?

Ernst: actually it is 4. In https://docs.gitlab.com/ee/gitlab-geo/database.html#step-1-configure-the-primary-server, step 11 it is checked on the primary. Then it is also done on the secondary in step 7 of https://docs.gitlab.com/ee/gitlab-geo/database.html#step-3-configure-the-secondary-server. Similarly there is some duplication of these instructions in database_source.md.

Task: dry this up a bit; can probably refer back to database from database_source, at least.