script: Use String::new() instead of the formatting infrastructure

when constructing DOM unique IDs.

This was showing up in the RoboHornet profile.

components/script/dom/node.rs

@@ -1156,7 +1156,7 @@ impl Node {
 
             layout_data: LayoutDataRef::new(),
 
-            unique_id: DOMRefCell::new("".to_string()),
+            unique_id: DOMRefCell::new(String::new()),
         }
     }