TrigCombinedEvent: Use [[maybe_unused]] instead of UNUSED macro.

Use standard [[maybe_unused]] instead of the non-standard UNUSED macro.

Trigger/TrigEvent/TrigCombinedEvent/CMakeLists.txt

@@ -13,7 +13,6 @@ atlas_depends_on_subdirs( PUBLIC
                           Trigger/TrigEvent/TrigNavigation
                           PRIVATE
                           AtlasTest/TestTools
-                          Control/CxxUtils
                           Control/AthenaKernel
                           Trigger/TrigDataAccess/TrigSerializeCnvSvc )
 

Trigger/TrigEvent/TrigCombinedEvent/src/TrigComposite.cxx

 /*
-  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2019 CERN for the benefit of the ATLAS collaboration
 */
 
 //#include <iostream>
@@ -7,7 +7,6 @@
 #include <stdexcept>
 #include <boost/foreach.hpp>	
 #include "TrigCombinedEvent/TrigComposite.h"
-#include "CxxUtils/unused.h"
 
 
 using namespace std;
@@ -136,14 +135,16 @@ void TrigComposite::eraseDetail(const std::string& key) {
 template<typename T>
 std::map<std::string, T>& TrigComposite::detailsMap() {
   // this should never be needed, need to add compile error here
-  int UNUSED(z) =
+  [[maybe_unused]]
+  int z =
     sizeof(struct TrigComposite_does_not_support_that_type_as_a_detail);
 }
 
 template<typename T>
 const std::map<std::string, T>& TrigComposite::detailsMap() const {
   // this should never be needed, need to add compile error here
-  int UNUSED(z) =
+  [[maybe_unused]]
+  int z =
     sizeof(struct TrigComposite_does_not_support_that_type_as_a_detail);
 
 }