Remove a TODO from the implementation of getComputedStyle

The spec is now clear that disconnected elements shouldn't return a
style.

Closes #6860.

components/script/dom/cssstyledeclaration.rs

@@ -242,8 +242,6 @@ impl CSSStyleDeclaration {
             CSSStyleOwner::Element(ref el) => {
                 let node = el.upcast::<Node>();
                 if !node.is_connected() {
-                    // TODO: Node should be matched against the style rules of this window.
-                    // Firefox is currently the only browser to implement this.
                     return DOMString::new();
                 }
                 let addr = node.to_trusted_node_address();