Revert "simplify sw unregister"

This reverts commit 27f8e1c7b6.

pages/index.js

@@ -46,7 +46,13 @@ class Index extends React.Component {
 function onReset() {
   clearSettings()
 
-  unregister()
+  if (window.navigator && navigator.serviceWorker) {
+    navigator.serviceWorker.getRegistrations().then(registrations => {
+      for (let registration of registrations) {
+        registration.unregister()
+      }
+    })
+  }
 }
 
 export default withRouter(Index)