Remove a bunch of unused functions from MathExtras.h.

These were functions used by JavaScriptCore (and, sometimes, by nobody at
all) in WebKit, so it's safe to get rid of them now.

R=thakis@chromium.org,jochen@chromium.org

Review URL: https://chromiumcodereview.appspot.com/19159004

git-svn-id: svn://svn.chromium.org/blink/trunk@154241 bbb929c8-8fbe-4397-9dbb-9b2b20218538