6lowpan: remove unnecessary set of headers

On receiving side we don't need to set any headers in skb because the
6LoWPAN layer do not access it. Currently these values will set twice
after calling netif_rx.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Reviewed-by: Werner Almesberger <werner@almesberger.net>
Signed-off-by: David S. Miller <davem@davemloft.net>