Remove a commented mutable data member of TRT_DriftCircleOnTrack

http://goetz.web.cern.ch/goetz/mutable_2019-05-11.html#InnerDetector_TRT

 nightly/master/2019-05-11T2151:InnerDetector/InDetRecEvent/InDetRIO_OnTrack/InDetRIO_OnTrack/TRT_DriftCircleOnTrack.h +170


   170	      
   171	      /** the contained RIO (PRD, PrepRawData) - TRT_DriftCircle in this case */
   172	//       mutable const InDet::TRT_DriftCircle*  m_rio;
   173	
   174	      ElementLinkToIDCTRT_DriftCircleContainer m_rio;