Remove SnapshotViewIOS from public interface (react#23497)

Summary:
Part of Lean Core react#23313

Removes `SnapshotViewIOS` from the public RN interface.

I think there's a wider discussion to be had here about whether `RCTTest` should be part of the public distribution or at least whether this should be split into a separate utils package. It's mainly used by the RNTester app. It seems to be little known about but there are [some references to it online](https://blog.callstack.io/testing-your-react-native-apps-abfe41903dfd).

[iOS] [Removed] - `SnapshotViewIOS` is no longer publicly exported from RN
Pull Request resolved: react#23497

Differential Revision: D14123280

Pulled By: cpojer

fbshipit-source-id: badaf6cb5d2195268f0f8b429fc11d6525747708

Author: jdmunro

Libraries/react-native/react-native-implementation.js

@@ -124,9 +124,6 @@ module.exports = {
     );
     return require('Slider');
   },
-  get SnapshotViewIOS() {
-    return require('SnapshotViewIOS');
-  },
   get Switch() {
     return require('Switch');
   },

RNTester/js/RNTesterApp.ios.js

@@ -21,12 +21,12 @@ const RNTesterExampleContainer = require('./RNTesterExampleContainer');
 const RNTesterExampleList = require('./RNTesterExampleList');
 const RNTesterList = require('./RNTesterList.ios');
 const RNTesterNavigationReducer = require('./RNTesterNavigationReducer');
+const SnapshotViewIOS = require('./SnapshotViewIOS.ios');
 const URIActionMap = require('./URIActionMap');
 
 const {
   Button,
   AppRegistry,
-  SnapshotViewIOS,
   StyleSheet,
   Text,
   View,