Drop unnecessary `_KEEP` and just `None` instead
    
    Per @jskladan, this was intended to allow passing of `None` to
    have a special meaning, but that was never really implemented
    and it doesn't seem likely to happen, so let's just do what is
    otherwise natural and use `None` here.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>