sit: remove unnecessary protocol check in ipip6_tunnel_xmit()

ipip6_tunnel_xmit() is called immediately after checking that
skb->protocol is  htons(ETH_P_IPV6) so there is no need
to check it a second time.

Found by inspection.

Signed-off-by: Simon Horman <simon.horman@netronome.com>
Reviewed-by: Dinan Gunawardena <dinan.gunawardena@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>