qeth: avoid useless removal of multicast addresses

Function qeth_l2_remove_device invokes qeth_l2_del_all_mc at the end.
This is needless, because it is already called in the offline function.
And even more this is invalid, because multicast addresses cannot be
removed in DOWN state. Thus this patch deletes invocation of
qeth_l2_del_all_mc in function qeth_l2_remove_device.

Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>