6lowpan: remove ipv6 module request

Since we use exported function from ipv6 kernel module we don't need to
request the module anymore to have ipv6 functionality.

Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <aar@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>