Revert "Add a way to disable running service workers. (#1167)"

This reverts commit d8af41d690.

lib/router/router.js

@@ -10,16 +10,6 @@ import { loadGetInitialProps, getLocationOrigin } from '../utils'
 // Add "fetch" polyfill for older browsers
 if (typeof window !== 'undefined') {
   require('whatwg-fetch')
-  // Unregister if there's an exisiting service worker.
-  // This is because we ship a service worker in previous version.
-  // We don't use it and having a already running worker might cause issues.
-  // (We can remove this in future releases)
-  navigator.serviceWorker.getRegistrations()
-    .then((registrations) => {
-      for (let registration of registrations) {
-        registration.unregister()
-      }
-    })
 }
 
 export default class Router extends EventEmitter {