Remove unused forward declaration of G4FastSimulationModel class

The correct class name is `G4VFastSimulationModel` and is included
via `FastSimulationBase.h`.

LArCalorimeter/LArG4/LArG4FastSimulation/src/LArFastShowerTool.h

 /*
-  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2020 CERN for the benefit of the ATLAS collaboration
 */
 
 #ifndef LARG4FASTSIMULATION_LARFASTSHOWERTOOL_H
@@ -16,8 +16,6 @@
 // STL headers
 #include <string>
 
-class G4FastSimulationModel;
-
 /// NEEDS DOCUMENTATION
 class LArFastShowerTool: public FastSimulationBase
 {