Clear event listeners by setting them to null, instead of function() {}.

This reduces the number of listeners shown in the Chrome DevTools timeline.

R=tyoshino@chromium.org

Review URL: https://codereview.appspot.com/136530044

git-svn-id: https://pywebsocket.googlecode.com/svn/trunk/src@846 4ff78f4a-9131-11de-b045-6380ec9940d4