Remove confusing self-link from mojo/README

The link at the top of this README claimed that it was a subset of itself.
Links to the EDK and C system documentation are a little more useful :)

R=rockot@chromium.org

R: rockot@chromium.org
Change-Id: I314fadea0f40e8b999c03b7414862b0e1602ed1e
Reviewed-on: https://chromium-review.googlesource.com/985241
Commit-Queue: Jeremy Apthorp <jeremya@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547498}

mojo/README.md

 # Mojo
-This document is a subset of the [Mojo documentation](/mojo).
 
 [TOC]