BREAKING kill deprecated asset support

Summary:
This removes support for `require('image!…')`, which has been deprecated for a long time.

It is still possible to use images that are already bundled by the native app using the `nativeImageSource` module.
Check http://facebook.github.io/react-native/docs/images.html for detailed documentation.

Reviewed By: matryoshcow

Differential Revision: D4231208

fbshipit-source-id: 05ec4c1ca0fabdc3fbb652f8ad1acdf240a67955