LibWeb: Remove unused code for constructing partial layout trees

We only ever build the whole layout tree in one go. Maybe one day we'll
support partial rebuilds, but for now, let's keep things simple.

Userland/Libraries/LibWeb/Layout/TreeBuilder.cpp

@@ -224,11 +224,7 @@ void TreeBuilder::create_layout_tree(DOM::Node& dom_node, TreeBuilder::Context&
 
 RefPtr<Node> TreeBuilder::build(DOM::Node& dom_node)
 {
-    if (dom_node.parent()) {
-        // We're building a partial layout tree, so start by building up the stack of parent layout nodes.
-        for (auto* ancestor = dom_node.parent()->layout_node(); ancestor; ancestor = ancestor->parent())
-            m_parent_stack.prepend(verify_cast<NodeWithStyle>(ancestor));
-    }
+    VERIFY(dom_node.is_document());
 
     Context context;
     create_layout_tree(dom_node, context);
@@ -263,8 +259,6 @@ void TreeBuilder::for_each_in_tree_with_inside_display(NodeWithStyle& root, Call
 
 void TreeBuilder::fixup_tables(NodeWithStyle& root)
 {
-    // NOTE: Even if we only do a partial build, we always do fixup from the root.
-
     remove_irrelevant_boxes(root);
     generate_missing_child_wrappers(root);
     generate_missing_parents(root);