stylo: Remove a lot of the restyle damage related complexity.

The only reason why we had the `existing_style_for_style_damage` bit is to apply
some optimizations that we don't have anymore.

I still want to reintroduce a few of them, at least for the non-eager
pseudo-element case... But I think I won't need this at all.

This allows us to remove a fair amount of Gecko code too.

components/layout_thread/dom_wrapper.rs

@@ -444,14 +444,6 @@ impl<'le> TElement for ServoLayoutElement<'le> {
         }
     }
 
-    #[inline]
-    fn existing_style_for_restyle_damage<'a>(&'a self,
-                                             current_cv: &'a ComputedValues,
-                                             _pseudo_element: Option<&PseudoElement>)
-                                             -> Option<&'a ComputedValues> {
-        Some(current_cv)
-    }
-
     fn has_dirty_descendants(&self) -> bool {
         unsafe { self.as_node().node.get_flag(HAS_DIRTY_DESCENDANTS) }
     }