Attempt to fix a crash in Scheduler::uiManagerDidCloneShadowNode

Summary:
changelog: [internal]

Attempt at fixing a crash in `Scheduler::uiManagerDidCloneShadowNode` by setting delegate to nullptr explicitly.

I don't think this will make a difference because `scheduler_` is released at the end of `dealloc` but it is worth a shot. Maybe some complex interaction between Obj-C and C++ comes into play here.

In this diff I also removed `_animationDriver = nullptr` because dealloc will do that automatically.

Reviewed By: philIip

Differential Revision: D32720901

fbshipit-source-id: 227ced2331384c47e8d15a323ee8a621bbb3d179

React/Fabric/RCTScheduler.mm

@@ -155,7 +155,8 @@ - (void)dealloc
   if (_animationDriver) {
     _animationDriver->setLayoutAnimationStatusDelegate(nullptr);
   }
-  _animationDriver = nullptr;
+
+  _scheduler->setDelegate(nullptr);
 }
 
 - (void)registerSurface:(facebook::react::SurfaceHandler const &)surfaceHandler