net: drop redundant ifdown/up of bond's nics.

Commit 5cabaaa130 has introduced a logic that we are trying to
eliminate: we should not ifdown a bonded nic directly, initscripts do it
for us.

Change-Id: Ia87fe1327448f3d859372be00b9ea8cd19d62e50
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/10584
Reviewed-by: Igor Lvovsky <ilvovsky@redhat.com>