ipv6: remove unneeded check to pskb_may_pull in ipip6_rcv

This is already checked by the caller (tunnel64_rcv) and brings ipip6_rcv
in line with ipip_rcv.

Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>