Simplify ping and pong

The simple demo apps grew to the point where they were no longer simple
and easy to understand since I was using them to test out timing reports
and accidentally committed the changes.  Restore the simplicity.

Change-Id: I453fa0351247c4549098660cd21191659269f515
diff --git a/aos/events/ping_lib.h b/aos/events/ping_lib.h
index 6be75ed..e14c3f2 100644
--- a/aos/events/ping_lib.h
+++ b/aos/events/ping_lib.h
@@ -25,10 +25,10 @@
   aos::Sender<examples::Ping> sender_;
   // Timer handle which sends the Ping message.
   aos::TimerHandler *timer_handle_;
-  aos::PhasedLoopHandler *phased_loop_handle_;
-  aos::Fetcher<examples::Pong> pong_fetcher_;
   // Number of pings sent.
   int count_ = 0;
+  // Last pong value received so we can detect missed pongs.
+  int last_pong_value_ = 0;
 };
 
 }  // namespace aos