LibWeb: Remove unused Document::on_layout_updated hook

Libraries/LibWeb/DOM/Document.cpp

@@ -262,8 +262,6 @@ void Document::update_layout()
         return;
 
     layout();
-    if (on_layout_updated)
-        on_layout_updated();
 }
 
 RefPtr<LayoutNode> Document::create_layout_node(const StyleProperties*) const

Libraries/LibWeb/DOM/Document.h

@@ -109,7 +109,6 @@ public:
 
     void update_style();
     void update_layout();
-    Function<void()> on_layout_updated;
 
     virtual bool is_child_allowed(const Node&) const override;