Remove unnecessary await in push handler

We were `await`ing a synchronous method here, which will have the effect
of deferring its execution, and thus also the `event.waitUntil` call. We
want the latter to happen before control returns from the event.
Removing the unnecessary `await` solves this.

This seems unlikely to be a problem in practice, but all the same, it's
not right :)

app/views/pwa/service_worker.js

@@ -9,8 +9,8 @@ self.addEventListener('fetch', (event) => {
   }
 })
 
-self.addEventListener("push", async (event) => {
-  const data = await event.data.json()
+self.addEventListener("push", (event) => {
+  const data = event.data.json()
   event.waitUntil(Promise.all([ showNotification(data), updateBadgeCount(data.options) ]))
 })