Documentation : Geo: Remove support for using Geo with an installation from source

https://docs.gitlab.com/ee/administration/geo/replication/#using-gitlab-installed-from-source-deprecated

Note: In GitLab 11.5, support for using Geo in GitLab source installations was deprecated and will be removed in a future release. 
Please consider migrating to GitLab Omnibus install.

We need to remove this support now that the deprecation notice has been in place for quite a few releases.