Remove vtkCleanPolyData filter

Due to the vtkPolyDataFilter, the output polydata can be modified and
remove tetrahedrons for lines or triangles. This causes issues when running
the FEM Sofa code so we need to make sure that all the cells are
tetrahedrons.
Historically, the clean filter was used to remove the vertices outside of
the labelmap (air and outside generated by cleaver) but since these labels
are now automatically filtered out when adding the cells, the clean filter
isn't needed anymore anyway.