remove unused and misleading function

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Closes https://github.com/facebook/react-native/pull/6867

Differential Revision: D3186273

Pulled By: ericvicenti

fb-gh-sync-id: 35e82fe4389a271a30c49dd5a3bcbab7ac29a0cd
fbshipit-source-id: 35e82fe4389a271a30c49dd5a3bcbab7ac29a0cd

Examples/UIExplorer/NavigationExperimental/NavigationCardStackExample.js

@@ -143,11 +143,6 @@ class NavigationCardStackExample extends React.Component {
     });
   }
 
-  _onNavigate(action) {
-    if (action && action.type === 'back') {
-      this._pop();
-    }
-  }
 }
 
 const styles = StyleSheet.create({