Remove the MathML stylesheet

We removed support for MathML but left the stylesheet which should
be pretty much dead code by now.

Review URL: https://chromiumcodereview.appspot.com/19734005

git-svn-id: svn://svn.chromium.org/blink/trunk@154689 bbb929c8-8fbe-4397-9dbb-9b2b20218538