Remove unused variable in HelloNavigation example

Summary:
Because it's just unused :)
Closes https://github.com/facebook/react-native/pull/16294

Differential Revision: D6030121

Pulled By: ericnakagawa

fbshipit-source-id: c0e9ad75413cb2ab6fbf1b46517a033f36a627a7

local-cli/templates/HelloNavigation/components/KeyboardSpacer.js

@@ -9,7 +9,6 @@ import {
   View,
   Keyboard,
   LayoutAnimation,
-  UIManager,
 } from 'react-native';
 
 type Props = {