Remove mousewheel event name workaround

Fixed upstream https://github.com/google/closure-library/pull/308

src/ol/map.js

@@ -265,8 +265,7 @@ ol.Map = function(options) {
     goog.events.EventType.TOUCHSTART,
     goog.events.EventType.MSPOINTERDOWN,
     ol.MapBrowserEvent.EventType.POINTERDOWN,
-    // see https://github.com/google/closure-library/pull/308
-    goog.userAgent.GECKO ? 'DOMMouseScroll' : 'mousewheel'
+    goog.events.EventType.MOUSEWHEEL
   ], goog.events.Event.stopPropagation);
   goog.dom.appendChild(this.viewport_, this.overlayContainerStopEvent_);