ipv4: remove useless codes in ipmr_device_event()

Commit 7dc00c82

 added a 'notify' parameter for vif_delete() to
distinguish whether to unregister the device.

When notify=1 means we does not need to unregister the device,
so calling unregister_netdevice_many is useless.

Signed-off-by: RongQing.Li <roy.qing.li@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>