chore: delete unused code

Author: LucasXu0

frontend/app_flowy/packages/flowy_editor/lib/document/node.dart

@@ -84,13 +84,6 @@ class Node extends ChangeNotifier with LinkedListEntry<Node> {
     return childAtIndex(path.first)?.childAtPath(path.sublist(1));
   }
 
-  Node root() {
-    if (parent != null) {
-      return parent!.root();
-    }
-    return this;
-  }
-
   Path path([Path previous = const []]) {
     if (parent == null) {
       return previous;

frontend/app_flowy/packages/flowy_editor/lib/document/state_tree.dart

@@ -16,12 +16,6 @@ class StateTree {
     return StateTree(root: root);
   }
 
-  // Path pathForNode(Node node) {
-  //   var nodeRoot = node.root();
-  //   assert(nodeRoot == root, "Every node's root must be same as root");
-
-  // }
-
   Node? nodeAtPath(Path path) {
     return root.childAtPath(path);
   }