Remove the ability to run the sheriff-o-matic tests without karma.

This doesn't work with dev_appserver and maintaining the list of
unittest files is a pain. In retrospect, this doesn't really add
much value.

Also, update the README with simpler instructions for installing
Node.

R=michaelpg@chromium.org

Review URL: https://codereview.chromium.org/617183006

git-svn-id: svn://svn.chromium.org/blink/trunk@183113 bbb929c8-8fbe-4397-9dbb-9b2b20218538