Revert gerrithub usage instructions.

Gerrithub requires overly broad write permissions, and hence its usage
is discontinued.

CONTRIBUTING

@@ -2,21 +2,3 @@
 Before sending a patch or Pull request, please fill out a Google CLA, using the following form:
 
   https://cla.developers.google.com/clas
-
-For complex changes, please use Gerrit:
-
-* Connect your github account with gerrithub, at
-
-  https://review.gerrithub.io/static/intro.html
-
-* The signup will setup your SSH keys from Github.
-
-* Create your change as a commit
-
-* Add an ID to the commit message:
-
-  echo "Change-Id: I"$(head -c 20 /dev/urandom | sha1sum | awk '{print $1}')
-
-* Push the change for review,
-
-  git push ssh://$USER@review.gerrithub.io:29418/hanwen/go-fuse HEAD:refs/for/master