Merge branch 'gcc9.AthenaMonitoring-20181213' into 'master'

AthenaMonitoring: Fix gcc9 warnings.

See merge request atlas/athena!16743

Control/AthenaMonitoring/src/AthMonBench.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
 */
 
 
@@ -30,7 +30,7 @@ public:
   static const MSG::Level s_resourceMonThreshold = MSG::DEBUG;
 
   AthMonBench();
-  ~AthMonBench(){}
+  ~AthMonBench() = default;
 
   //Modify:
   void reset();