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)
     }

components/style/dom.rs

@@ -100,8 +100,6 @@ pub trait TNode : Sized + Copy + Clone {
 
     fn as_document(&self) -> Option<Self::ConcreteDocument>;
 
-    fn children_count(&self) -> u32;
-
     fn has_changed(&self) -> bool;
 
     unsafe fn set_changed(&self, value: bool);

ports/geckolib/wrapper.rs

@@ -5,7 +5,6 @@
 #![allow(unsafe_code)]
 
 use gecko_bindings::bindings;
-use gecko_bindings::bindings::Gecko_ChildrenCount;
 use gecko_bindings::bindings::Gecko_ClassOrClassList;
 use gecko_bindings::bindings::Gecko_GetNodeData;
 use gecko_bindings::bindings::Gecko_GetStyleContext;
@@ -187,12 +186,6 @@ impl<'ln> TNode for GeckoNode<'ln> {
         unimplemented!()
     }
 
-    fn children_count(&self) -> u32 {
-        unsafe {
-            Gecko_ChildrenCount(self.node)
-        }
-    }
-
     fn as_element(&self) -> Option<GeckoElement<'ln>> {
         if self.is_element() {
             unsafe { Some(GeckoElement::from_raw(self.node as *mut RawGeckoElement)) }