Remove mutable cache variables to fix constness issues of PixelDistortionsTool

Remove mutable cache variables to fix constness issues of PixelDistortionsTool

http://goetz.web.cern.ch/goetz/mutable_2019-07-13.html#InnerDetector_Pixel

 nightly/master/2019-07-13T2149:InnerDetector/InDetConditions/PixelConditionsTools/src/PixelDistortionsTool.h +110


   108	  bool m_doCorrections;
   109	
   110	  mutable const float * m_lastDisto;
   111	  mutable Identifier m_lastID;
   112	

nightly/master/2019-07-13T2149:InnerDetector/InDetConditions/PixelConditionsTools/src/PixelDistortionsTool.h +111


   109	
   110	  mutable const float * m_lastDisto;
   111	  mutable Identifier m_lastID;
   112	
   113	  //StoreGateSvc * m_detStore;

nightly/master/2019-07-13T2149:InnerDetector/InDetConditions/PixelConditionsTools/src/PixelDistortionsTool.h +114


   112	
   113	  //StoreGateSvc * m_detStore;
   114	  //mutable MsgStream m_log;
   115	
   116	  bool m_ownDistortions;