make test pass (react-navigation#1253)

test fails since react-navigation#1231
`383:25  error    'options' is assigned a value but never used  no-unused-vars`

Author: nihgwu

src/views/CardStack.js

@@ -380,7 +380,7 @@ class CardStack extends Component {
     SceneComponent: ReactClass<*>,
     scene: NavigationScene,
   ): React.Element<any> {
-    const { navigation, options } = this._getScreenDetails(scene);
+    const { navigation } = this._getScreenDetails(scene);
     const { screenProps } = this.props;
     const headerMode = this._getHeaderMode();
     if (headerMode === 'screen') {