ref(modules/UI): remove events system.

Many of the events are not used at all or used only on one place. For the rest of them the listeners were added 2 times on promoted visitors and not cleaned at all.

react/features/conference/components/web/Conference.tsx

@@ -368,8 +368,6 @@ class Conference extends AbstractConference<IProps, any> {
      */
     _start() {
         APP.UI.start();
-
-        APP.UI.registerListeners();
         APP.UI.bindEvents();
 
         FULL_SCREEN_EVENTS.forEach(name =>