Remove dead link from "External Editors" documentation
Summary: Fixes T12418. This is a fairly advanced feature and I think users can reasonably consult the documentation for their own editors to figure out how to do this.
Test Plan: Saw no more text.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T12418
Differential Revision: https://secure.phabricator.com/D17510