Revert "Set the public path to the asset manifest entries (facebook#2544)"

This reverts commit bab2c29.

I meant to apply it to `next` instead.

Author: gaearon

config/webpack.config.prod.js

@@ -319,7 +319,6 @@ module.exports = {
     // having to parse `index.html`.
     new ManifestPlugin({
       fileName: 'asset-manifest.json',
-      publicPath: publicPath
     }),
     // Generate a service worker script that will precache, and keep up to date,
     // the HTML & assets that are part of the Webpack build.