bonding: clean up bond_del_vlan()

1) the setting of NETIF_F_VLAN_CHALLENGED in bond_del_vlan() is
useless since commit b2a103e6 because bond_fix_features() now
sets NETIF_F_VLAN_CHALLENGED whenever the last slave is being
removed.

2) the code never triggers anyway as vlan_list is never empty
since ad1afb00.

Signed-off-by: Jiri Bohac <jbohac@suse.cz>
Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>