PanTauEvent: 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.

Reconstruction/PanTau/PanTauEvent/PanTauEvent/TauFeature.h

 /*
-  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2018 CERN for the benefit of the ATLAS collaboration
 */
 
 ///////////////////////////////////////////////////////////////////
@@ -59,7 +59,7 @@ public:
 //         );
 
     /** Destructor */
-    virtual ~TauFeature();
+    ~TauFeature() = default;
 
     /** returns the value of the feature given by its name*/
     double value(const std::string& ItsName, bool& isValid) const;

Reconstruction/PanTau/PanTauEvent/src/TauFeature.cxx

 /*
-  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2018 CERN for the benefit of the ATLAS collaboration
 */
 
 #include "PanTauEvent/TauFeature.h"
@@ -23,11 +23,6 @@ PanTau::TauFeature::TauFeature()
 
 
 
-PanTau::TauFeature::~TauFeature()
-{
-}
-
-
 double PanTau::TauFeature::value(const std::string& ItsName, bool& isValid) const
 {
     FeatureMapConstIter iter = m_featureMap.find(ItsName);