Remove HAS_DIRTY_SIBLINGS.

This isn't doing anything right now, and we're not even setting it properly
in dirty_impl the |dirty_subtree(self)| was causing us to hit the skip case
for step 3.

components/layout/layout_task.rs

@@ -1491,7 +1491,6 @@ impl LayoutTask {
             // "changed":
             // > node.set_changed(true);
             node.set_dirty(true);
-            node.set_dirty_siblings(true);
             node.set_dirty_descendants(true);
         }
     }