remove extra argument to unbindProperty that was ignored

[jmesserly]

unregisterObserver only takes a "name" argument:

  function unregisterObserver(element, name) {
    var o$ = getElementObservers(element);
    if (o$ && o$[name]) {
      o$[name].close();
      o$[name] = null;
      return true;
    }
  }

[sorvell]

LGTM, thx.