AnalysisTriggerAlgs: Fix clang warning.

clang warning: unused variable.

PhysicsAnalysis/AnalysisTrigger/AnalysisTriggerAlgs/src/RoIBResultToxAOD.cxx

 /*
-  Copyright (C) 2002-2018 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2019 CERN for the benefit of the ATLAS collaboration
 */
 //
 //
@@ -42,9 +42,6 @@ namespace {
    /// Giga-electronvolts
    static const double GeV = 1000.0;
 
-   /// Multiplier when we want to divide by GeV
-   static const double InvGeV = 1 / GeV;
-
 } // private namespace
 
 /// Helper macro for initialising SG keys that are allowed to be missing