Delete SampleApp

Summary: The sample app's code has been converted to Yeoman generator templates. Tell people to run the UIExplorer instead, or the `react-native init` project (which is the same as the SampleApp). This will slightly reduce the size of the npm package and cleans up unused files.
Closes https://github.com/facebook/react-native/pull/3025

Reviewed By: @​svcscm

Differential Revision: D2484901

Pulled By: @foghina