Remove hg config from dotfiles

Summary:
this only affects new users (everyone else can keep using it), but it
basically just stops us installing hg, hg configs, hg hooks and hg certs which
we don't need anymore now that we're mostly using git for everything especially
now that we're mostly using git for everything. new users are spared another
set of confusing config we don't actively use.

Test Plan: fingers crossed

Reviewers: chris

Reviewed By: chris

Differential Revision: http://phabricator.khanacademy.org/D9048