fix NativeAnimated event listener bug

Summary:
This bugfix cleans up reference to value listener upon unsubscribing to native events ultimately preventing RCTEventEmitter from throwing a [listener count error](https://github.com/facebook/react-native/blob/v0.33.0-rc.0/React/Modules/RCTEventEmitter.m#L90):

`Attempted to remove more RCTNativeAnimatedModule listeners than added`

/cc janicduplessis sahrens

🍺
Closes https://github.com/facebook/react-native/pull/9729

Differential Revision: D3818241

Pulled By: sahrens

fbshipit-source-id: e235e3f97534ef2a50df29877ce36d3b6095237e

Libraries/Animated/src/AnimatedImplementation.js

@@ -788,6 +788,7 @@ class AnimatedValue extends AnimatedWithChildren {
     }
 
     this.__nativeAnimatedValueListener.remove();
+    this.__nativeAnimatedValueListener = null;
     NativeAnimatedAPI.stopListeningToAnimatedNodeValue(this.__getNativeTag());
   }