staging: dgrp: remove redundant NULL check before unregister_dgrp_device().

unregister_dgrp_device on a NULL pointer is a no-op, so the NULL checks in
dgrp_remove_nd() can be removed.

Signed-off-by: Cyril Roelandt <tipecaml@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>