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