PingPong example: let the parent QObject do the cleanup

The m_timer and m_serverInfo instances are created using 'this' as a
parent, so we do not need to explicitly delete them in the destructor,
but let the parent object deal with them.
This also prevents a crash at application close.

Pick-to: 6.3 6.2 5.15
Change-Id: If91f01e1ab6dc8e839a06773464eeb9858d2fd36
Reviewed-by: Juha Vuolle <juha.vuolle@insta.fi>
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>

examples/bluetooth/pingpong/pingpong.cpp

@@ -60,8 +60,7 @@ PingPong::PingPong()
 
 PingPong::~PingPong()
 {
-    delete m_timer;
-    delete m_serverInfo;
+    m_timer->stop();
     delete socket;
     delete discoveryAgent;
 }