style: Get rid of unstyled children only traversals.

They're useless now, provided we remove the hack to not traverse XBL-bound
elements on initial styling.

Bug: 1418456
Reviewed-by: heycam
MozReview-Commit-ID: AvBVdyF1wb6

components/style/dom.rs

@@ -557,11 +557,6 @@ pub trait TElement
                    !data.hint.has_animation_hint_or_recascade();
         }
 
-        if traversal_flags.contains(TraversalFlags::UnstyledOnly) {
-            // We don't process invalidations in UnstyledOnly mode.
-            return data.has_styles();
-        }
-
         if self.has_snapshot() && !self.handled_snapshot() {
             return false;
         }