Bug 1424782 - Follow-up to remove overzealous assertion I added. r=me

With webrender and gfx.webrender.hit-test enabled, we create
nsDisplayCompositorHitTestInfo items which are subclasses of nsDisplayEventReceiver,
and we do so even for painting display lists. So this assertion trips even though
this is the desired behaviour. I'm taking the assertion out as it is not really
needed.

DONTBUILD because trivial assertion removal

MozReview-Commit-ID: Bs9PjtQSwqQ

layout/painting/nsDisplayList.h

@@ -4307,7 +4307,6 @@ public:
   nsDisplayEventReceiver(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame)
     : nsDisplayItem(aBuilder, aFrame) {
     MOZ_COUNT_CTOR(nsDisplayEventReceiver);
-    MOZ_ASSERT(aBuilder->IsForEventDelivery());
   }
 #ifdef NS_BUILD_REFCNT_LOGGING
   virtual ~nsDisplayEventReceiver() {