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>

net/ipv6/sit.c

@@ -825,9 +825,6 @@ static netdev_tx_t ipip6_tunnel_xmit(struct sk_buff *skb,
 	u8 protocol = IPPROTO_IPV6;
 	int t_hlen = tunnel->hlen + sizeof(struct iphdr);
 
-	if (skb->protocol != htons(ETH_P_IPV6))
-		goto tx_error;
-
 	if (tos == 1)
 		tos = ipv6_get_dsfield(iph6);