android: Remove a check on applicationContext's type in FcmListenerService

As Greg points out in
  #5695 (comment) :

> This logic is a bit puzzling; it made me wonder why we have it.
> The one line of the function after this doesn't have an obvious
> reason to care about the type of the `applicationContext`.
>
> It looks like the answer is that this function used to interact
> with `applicationContext`, until eaa8b40. So until then, this was
> needed in order for that to type-check.
>
> Now we can just take it out.

Author: chrisbobbe

android/app/src/main/java/com/zulipmobile/notifications/FcmListenerService.java

@@ -11,10 +11,6 @@
 public class FcmListenerService extends FirebaseMessagingService {
     @Override
     public void onMessageReceived(RemoteMessage message) {
-        final Context applicationContext = getApplicationContext();
-        if (!(applicationContext instanceof MainApplication)) {
-            return;
-        }
         NotificationUiManager.onReceived(this, message.getData());
     }