mrstats: removed duplicate ignorezero, kept the one in stats.cpp
mrstats: removed duplicate ignorezero, kept the one in stats.cpp

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3