Remove gist link that (never?) doesn't work

Summary: Removing 404 link, fixes https://github.com/phacility/arcanist/issues/180

Test Plan: n/a

Reviewers: epriestley, avivey

Reviewed By: epriestley, avivey

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D10689
Patch: don't complain about reviewers not being specified.
This will break in the future when upstream arc incorporates T4631. Hopefully there will be a better solution by then
if you didn't specify a reviewer, suggest projects