Remove const_cast from RingerRingsCnv_p1

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

nightly/master/2019-07-13T2149:Trigger/TrigEvent/TrigCaloEventTPCnv/src/RingerRingsCnv_p1.cxx +20

    18	
    19	  // Hack! Should be fixed by adding a const accessor on RingerRings
    20	  RingerRings *trans_non_const = const_cast<RingerRings *>(trans);
    21	  
    22	  pers->m_numberOfRings = trans->size();
nightly/master/2019-07-13T2149:Trigger/TrigEvent/TrigCaloEventTPCnv/src/RingerRingsCnv_p2.cxx +19

    17	
    18	  // Hack! Should be fixed by adding a const accessor on RingerRings
    19	  //RingerRings *trans_non_const = const_cast<RingerRings *>(trans);
    20	  
    21	  pers->m_numberOfRings = trans->size();