[effects/slidingpopups] Don't update property on window closed

If the property changes the animation data is updated. As the last
property change event will definately be delivered before an unmap notify
we can be sure that the animation data is up to data. Reading the
property again does not have any advantages.

By removing it we get rid of at least one roundtrip to the X Server and
it fixes one failed GetProperty error when it tries to read the property
for an already destroyed unmanaged.

REVIEW: 117431

effects/slidingpopups/slidingpopups.cpp

@@ -278,7 +278,6 @@ void SlidingPopupsEffect::slotWindowAdded(EffectWindow *w)
 
 void SlidingPopupsEffect::slotWindowClosed(EffectWindow* w)
 {
-    slotPropertyNotify(w, mAtom);
     if (w->isOnCurrentDesktop() && !w->isMinimized() && mWindowsData.contains(w)) {
         w->refWindow();
         delete mAppearingWindows.take(w);