AthExThinning: Fix gcc9 warnings.

In C++11, implicit declarations of copy and assignment are deprecated if the
class has a user defined destructor or copy or assignment.  gcc9 now warns
about this by default.
Adjust to avoid the warning.

Control/AthenaExamples/AthExThinning/AthExThinning/AthExIParticle.h

 ///////////////////////// -*- C++ -*- /////////////////////////////
 
 /*
-  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2018 CERN for the benefit of the ATLAS collaboration
 */
 
 // AthExIParticle.h 
@@ -27,7 +27,7 @@ class AthExIParticle
 
   /** Destructor: 
    */
-  virtual ~AthExIParticle();
+  virtual ~AthExIParticle() = default;
 
   /////////////////////////////////////////////////////////////////// 
   // Const methods: 

Control/AthenaExamples/AthExThinning/src_lib/AthExIParticle.cxx

 ///////////////////////// -*- C++ -*- /////////////////////////////
 
 /*
-  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2018 CERN for the benefit of the ATLAS collaboration
 */
 
 // AthExIParticle.cxx 
@@ -15,13 +15,6 @@
 // AthExThinning includes
 #include "AthExThinning/AthExIParticle.h"
 
-/////////////////////////////////////////////////////////////////// 
-// Public methods: 
-/////////////////////////////////////////////////////////////////// 
-
-AthExIParticle::~AthExIParticle()
-{}
-
 /////////////////////////////////////////////////////////////////// 
 // Const methods: 
 ///////////////////////////////////////////////////////////////////