clear private HOOK_ID as soon as hook is destroyed

Fixes #3496.

When navigating from a sticky LiveView to another page that contains
a hook on an element with the same id, morphdom would merge the old
element into the new one and still find the old HOOK_ID in the element's
DOM.private data, leading to the new hook not initalizing properly.

assets/js/phoenix_live_view/view_hook.js

@@ -33,7 +33,10 @@ export default class ViewHook {
   __mounted(){ this.mounted && this.mounted() }
   __updated(){ this.updated && this.updated() }
   __beforeUpdate(){ this.beforeUpdate && this.beforeUpdate() }
-  __destroyed(){ this.destroyed && this.destroyed() }
+  __destroyed(){
+    this.destroyed && this.destroyed()
+    DOM.deletePrivate(this.el, HOOK_ID) // https://github.com/phoenixframework/phoenix_live_view/issues/3496
+  }
   __reconnected(){
     if(this.__isDisconnected){
       this.__isDisconnected = false