Remove promise and fetch polyfills from browser build

Reduces bundle size by 23%
Only IE needs polyfilling these days, and there has always been warnings in the readme about required polyfills

package.json

@@ -63,7 +63,6 @@
     "codecov": "^2.3.0",
     "cross-env": "^5.0.1",
     "css-loader": "^0.28.0",
-    "es6-promise": "^4.0.3",
     "exports-loader": "^0.6.3",
     "extract-text-webpack-plugin": "^3.0.0",
     "html-webpack-plugin": "^2.30.1",

webpack.browser.babel.js

@@ -1,5 +1,4 @@
 import path from 'path'
-import webpack from 'webpack'
 import config, { minifyPlugins } from './webpack.production.babel'
 
 export default {
@@ -24,11 +23,5 @@ export default {
       }
     ]
   },
-  plugins: [
-    new webpack.ProvidePlugin({
-      'Promise': 'exports-loader?global.Promise!es6-promise',
-      'window.fetch': 'exports-loader?self.fetch!whatwg-fetch'
-    }),
-    ...minifyPlugins
-  ]
+  plugins: minifyPlugins
 }

yarn.lock

@@ -2276,10 +2276,6 @@ es6-map@^0.1.3:
     es6-symbol "~3.1.1"
     event-emitter "~0.3.5"
 
-es6-promise@^4.0.3:
-  version "4.1.1"
-  resolved "https://registry.yarnpkg.com/es6-promise/-/es6-promise-4.1.1.tgz#8811e90915d9a0dba36274f0b242dbda78f9c92a"
-
 es6-set@~0.1.5:
   version "0.1.5"
   resolved "https://registry.yarnpkg.com/es6-set/-/es6-set-0.1.5.tgz#d2b3ec5d4d800ced818db538d28974db0a73ccb1"