Deleted unnecessary 'use strict' in new project templates

Summary:Hello there!

Since React Native generates `index.*.js` files with usage of ES6 modules, `'use strict'` became unnecessary.

http://www.ecma-international.org/ecma-262/6.0/#sec-strict-mode-code
> Module code is always strict mode code.
Closes https://github.com/facebook/react-native/pull/6322

Differential Revision: D3017181

Pulled By: vjeux

fb-gh-sync-id: 39e0fe703b8d8a0093b952f2c18a36d0d28e1020
shipit-source-id: 39e0fe703b8d8a0093b952f2c18a36d0d28e1020

local-cli/generator/templates/index.android.js

@@ -2,7 +2,7 @@
  * Sample React Native App
  * https://github.com/facebook/react-native
  */
-'use strict';
+
 import React, {
   AppRegistry,
   Component,

local-cli/generator/templates/index.ios.js

@@ -2,7 +2,7 @@
  * Sample React Native App
  * https://github.com/facebook/react-native
  */
-'use strict';
+
 import React, {
   AppRegistry,
   Component,