Bluetooth: 6lowpan: Fix double kfree of netdev priv

This patch removes the kfree of the netdev priv in device_event() upon
NETDEV_UNREGISTER event. The freeing of memory is taken care of by the
netdev destructor.

Signed-off-by: Lukasz Duda <lukasz.duda@nordicsemi.no>
Signed-off-by: Glenn Ruben Bakke <glenn.ruben.bakke@nordicsemi.no>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>