Remove unused children_count method from TNode.

The semantics of this method with respect to anonymous children are unclear.
Thankfully it's unused, so we can remove it.

components/script/layout_wrapper.rs

@@ -170,10 +170,6 @@ impl<'ln> TNode for ServoLayoutNode<'ln> {
         self.opaque().0
     }
 
-    fn children_count(&self) -> u32 {
-        unsafe { self.node.children_count() }
-    }
-
     fn as_element(&self) -> Option<ServoLayoutElement<'ln>> {
         as_element(self.node)
     }