Clean unused import modules.

Summary:
Thanks for submitting a pull request! Please provide enough information so that others can review your pull request:

> **Unless you are a React Native release maintainer and cherry-picking an *existing* commit into a current release, ensure your pull request is targeting the `master` React Native branch.**

Explain the **motivation** for making this change. What existing problem does the pull request solve?

Prefer **small pull requests**. These are much easier to review and more likely to get merged. Make sure the PR does only one thing, otherwise please split it.

**Test plan (required)**

Demonstrate the code is solid. Example: The exact commands you ran and their output, screenshots / videos if the pull request changes UI.

Make sure tests pass on both Travis and Circle CI.

**Code formatting**

Look around. Match the style of the rest of the codebase. See also the simple [style guide](https://github.com/facebook/react-native/blob/master/CONTRIBUTING.md#style-guide).

For more info, see
Closes https://github.com/facebook/react-native/pull/10413

Differential Revision: D4028108

fbshipit-source-id: 99a864dfda578d640f582b296583591415ba26cd

Examples/UIExplorer/js/PickerIOSExample.js

@@ -122,8 +122,6 @@ class PickerStyleExample extends React.Component {
   };
 
   render() {
-    var make = CAR_MAKES_AND_MODELS[this.state.carMake];
-    var selectionString = make.name + ' ' + make.models[this.state.modelIndex];
     return (
       <PickerIOS
         itemStyle={{fontSize: 25, color: 'red', textAlign: 'left', fontWeight: 'bold'}}