multipath: remove unused device attribute 'devnum'

Change-Id: I033b54d73e5a7ded19202c48874db394407651e2
Signed-off-by: Yeela Kaplan <ykaplan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/13356
Reviewed-by: Dan Kenigsberg <danken@redhat.com>