Just as documented, pixel unit should always return factor 0. There is
no need to call _gimp_unit_vtable.unit_get_factor().

This is even more important as there is one implementation of
unit_get_factor() in core, and another in libgimp and the one in libgimp
is expecting unit to always be >= GIMP_UNIT_INCH. So we were getting
CRITICALs in libgimp when calling gimp_unit_get_factor() on pixel unit
(for instance when drawing a GimpRuler).