ensure a relation between template and content is remove while content is deleting

Author: cklei-carly

src/Observers/ContentObserver.php

@@ -127,6 +127,8 @@ public function forceDeleting($model)
         $model->path()->delete();
         $model->routes()->delete();
 
+        $model->templateable()->delete();
+
         $this->clearCached(); // Since the navigation is deleted, we need to clear the cache.
     }