[refactor] Delete meaningless code and clarify it

Author: kahirokunn

packages/notification/src/main.js

@@ -28,10 +28,10 @@ const Notification = function(options) {
     options.message = 'REPLACED_BY_VNODE';
   }
   instance.id = id;
-  instance.vm = instance.$mount();
-  document.body.appendChild(instance.vm.$el);
-  instance.vm.visible = true;
-  instance.dom = instance.vm.$el;
+  instance.$mount();
+  document.body.appendChild(instance.$el);
+  instance.visible = true;
+  instance.dom = instance.$el;
   instance.dom.style.zIndex = PopupManager.nextZIndex();
 
   let verticalOffset = options.offset || 0;
@@ -41,7 +41,7 @@ const Notification = function(options) {
   verticalOffset += 16;
   instance.verticalOffset = verticalOffset;
   instances.push(instance);
-  return instance.vm;
+  return instance;
 };
 
 ['success', 'warning', 'info', 'error'].forEach(type => {