Remove unneeded async (#4635)

Unless I'm missing something, we don't need this async. The transpiled JavaScript is much smaller 🚀

lib/router/router.js

@@ -53,7 +53,7 @@ export default class Router {
     }
   }
 
-  async onPopState (e) {
+  onPopState (e) {
     if (!e.state) {
       // We get state as undefined for two reasons.
       //  1. With older safari (< 8) and older chrome (< 34)