clean UIExplorer NavigationExperimental example code

Summary:
1. Explain the **motivation** for making this change.

    Those codes cleaned are not used anywhere, remove them would make the example much more clear to users.

2. **Test plan (required)**

    ![navigationexperimental](https://cloud.githubusercontent.com/assets/1091472/17392196/4f28cba4-5a4e-11e6-9ef1-727edc784f1f.gif)
Closes https://github.com/facebook/react-native/pull/9203

Differential Revision: D3668848

fbshipit-source-id: c604a21c84dba72a5a3c857fc16bc7febfe20377

Examples/UIExplorer/js/NavigationExperimental/NavigationTransitioner-AnimatedView-pager-example.js

@@ -137,8 +137,6 @@ class Example extends Component {
 class ExampleNavigator extends Component {
   _render: Function;
   _renderScene: Function;
-  _back: Function;
-  _forward: Function;
 
   props: {
     navigate: Function,
@@ -154,8 +152,6 @@ class ExampleNavigator extends Component {
     super(props, context);
     this._render = this._render.bind(this);
     this._renderScene = this._renderScene.bind(this);
-    this._back = this._back.bind(this);
-    this._forward = this._forward.bind(this);
   }
 
   render(): ReactElement<any> {
@@ -195,14 +191,6 @@ class ExampleNavigator extends Component {
       />
     );
   }
-
-  _back(): void {
-    this.props.navigate('back');
-  }
-
-  _forward(): void {
-    this.props.navigate('forward');
-  }
 }
 
 class ExampleScene extends Component {
@@ -261,10 +249,6 @@ const styles = StyleSheet.create({
     right: 0,
     top: 0,
   },
-  scrollView: {
-    flex: 1,
-    padding: 50,
-  },
   heading: {
     alignItems : 'center',
     flex: 1,