Remove unused forward declarations in IMDT_RDO_Decoder

Hi,

this MR removes unused forward declarations in IMDT_RDO_Decoder.

Best, Nico

MuonSpectrometer/MuonCnv/MuonMDT_CnvTools/MuonMDT_CnvTools/IMDT_RDO_Decoder.h

 /*
-  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2021 CERN for the benefit of the ATLAS collaboration
 */
 
 #ifndef MUONMDT_CNVTOOLS_IMDT_RDO_DECODER_H
@@ -8,8 +8,6 @@
 #include "GaudiKernel/IAlgTool.h"
 
 class MdtDigit;
-class MdtIdHelper;
-class MuonMDT_CablingSvc;
 class MdtAmtHit;
 class Identifier;
 
@@ -24,9 +22,7 @@ namespace Muon {
     /** AlgTool InterfaceID
      */
     static const InterfaceID& interfaceID( );
-    
-    //    virtual void set(const MdtIdHelper * mdtIdHelper, const MDTcablingSvc* mdtCabling)=0;
-    
+  
     virtual MdtDigit * getDigit(const MdtAmtHit * amtHit, uint16_t& subdetId, 
 				uint16_t& mrodId, uint16_t& csmId) const = 0;