Remove commented const_cast in xAODSlowMuonContainerCnv

http://goetz.web.cern.ch/goetz/mutable_2019-06-10.html#Cnv_Muon

nightly/master/2019-06-10T2153:Event/xAOD/xAODMuonAthenaPool/src/xAODSlowMuonContainerCnv.cxx +92

    90	toPersistent( xAOD::SlowMuon* /*slowMuon*/ ) const {
    91	  using namespace xAOD;
    92	  // const_cast<ElementLink< MuonContainer >&>(slowMuon->muonLink()).toPersistent();
    93	  return;
    94	}