Remove CSSDefaultStyleSheets::mathMLStyleSheet

The variable was never initialized nor used. While touching the code,
also removed more unused functions or stale references to MathML.

Review URL: https://chromiumcodereview.appspot.com/23819050

git-svn-id: svn://svn.chromium.org/blink/trunk@157637 bbb929c8-8fbe-4397-9dbb-9b2b20218538