fix: remove key error

Author: asjqkkkk

frontend/appflowy_flutter/lib/plugins/document/presentation/editor_plugins/desktop_toolbar/link/link_extension.dart

@@ -8,9 +8,9 @@ extension LinkExtension on EditorState {
     if (node == null) {
       return;
     }
-    final attributes = _getAttribute(node, selection)
-      ..remove(BuiltInAttributeKey.href)
-      ..remove(kIsPageLink);
+    final attributes = _getAttribute(node, selection);
+    attributes.remove(BuiltInAttributeKey.href);
+    attributes.remove(kIsPageLink);
     final index = selection.normalized.startIndex;
     final length = selection.length;
     final transaction = this.transaction
@@ -42,9 +42,9 @@ extension LinkExtension on EditorState {
     if (node == null) {
       return;
     }
-    final attributes = _getAttribute(node, selection)
-      ..remove(BuiltInAttributeKey.href)
-      ..remove(kIsPageLink);
+    final attributes = _getAttribute(node, selection);
+    attributes.remove(BuiltInAttributeKey.href);
+    attributes.remove(kIsPageLink);
     final index = selection.normalized.startIndex;
     final length = selection.length;
     final transaction = this.transaction