Remove a mutable member of IDAlignMonGenericTracks.

http://goetz.web.cern.ch/goetz/mutable_2019-07-13.html#Monitoring,Mon,DQ_Align

nightly/master/2019-07-13T2149:InnerDetector/InDetMonitoring/InDetAlignmentMonitoring/src/IDAlignMonGenericTracks.h +400
f1364ba73b236311fac5df3292c28f2c0158d179	2019-06-17_T1851	Susumu Oda	Remove a commented mutable data member of IDAlignMonGenericTracks
42309b79297ada009aab87e1dc5b864ae0d738d0	2019-02-02_T0111	Vakho Tsulaia	InDetPerformanceMonitoring migrated to xAOD::EventInfo

   398	  const xAOD::Vertex*      findAssociatedVertexTP(const xAOD::TrackParticle *) const;
   399	
   400	  mutable std::map<const xAOD::TrackParticle*, const xAOD::Vertex* > m_trackVertexMapTP;
   401	  
   402	  const AtlasDetectorID*                m_idHelper;