Unsubscribe from the channel when UpdatesForElement disconnects (#299)

Unsubscribe from the channel by calling superclass's (SubscribingElement) disconnectedCallback.

## Why should this be added
Fix repeated subscribe messages after a morph.

javascript/elements/updates_for_element.js

@@ -64,6 +64,7 @@ export default class UpdatesForElement extends SubscribingElement {
   }
 
   disconnectedCallback () {
+    super.disconnectedCallback()
     if (this.observeAppearance) {
       this.appearanceObserver.stop()
     }