build: Don't remove bin/mrtrix3.pyc
build: Don't remove bin/mrtrix3.pyc

Resolve conflict between #1550 and #1613, which resulted in compiled Python file "bin/mrtrix3.pyc" being moved to bin/purged_files/.

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