[ReactNative] Remove POPAnimation

Summary:
Thanks to @vjeux' work on Animated, we have transitioned away from POPAnimation entirely

Libraries/ReactIOS/NativeMethodsMixin.js

@@ -12,7 +12,6 @@
 'use strict';
 
 var NativeModules = require('NativeModules');
-var RCTPOPAnimationManager = NativeModules.POPAnimationManager;
 var RCTUIManager = NativeModules.UIManager;
 var TextInputState = require('TextInputState');
 
@@ -38,32 +37,8 @@ type MeasureLayoutOnSuccessCallback = (
   height: number
 ) => void
 
-var animationIDInvariant = function(
-  funcName: string,
-  anim: number
-) {
-  invariant(
-    anim,
-    funcName + ' must be called with a valid animation ID returned from' +
-    ' POPAnimation.createAnimation, received: "' + anim + '"'
-  );
-};
 
 var NativeMethodsMixin = {
-  addAnimation: function(anim: number, callback?: (finished: bool) => void) {
-    animationIDInvariant('addAnimation', anim);
-    RCTPOPAnimationManager.addAnimation(
-      findNodeHandle(this),
-      anim,
-      mountSafeCallback(this, callback)
-    );
-  },
-
-  removeAnimation: function(anim: number) {
-    animationIDInvariant('removeAnimation', anim);
-    RCTPOPAnimationManager.removeAnimation(findNodeHandle(this), anim);
-  },
-
   measure: function(callback: MeasureOnSuccessCallback) {
     RCTUIManager.measure(
       findNodeHandle(this),