This mode forces the selection to be empty. Although there is no
practical difference between having MODE_NONE and having MODE_SELECTED
with nothing selected (which is the default), this is a semantically
nicer way to say you don't want any fields.

Since we have changed the default behavior of Filter::Execute to be
MODE_ALL, this is a nice addition so that you can clearly specify you
don't want to pass any fields by adding
vtkm::filter::FieldSelection::MODE_NONE as the second argument. (Making
it MODE_SELECTED is not clear that you want none.)