ipv6: Remove sit_gro_receive()

This was backported as part of 066b300 ("tunnels: Don't apply GRO
to multiple layers of encapsulation.") but nothing uses this function,
thus resulting in this build warning:

net/ipv6/ip6_offload.c:264:25: warning: 'sit_gro_receive' defined but
not used [-Wunused-function]
 static struct sk_buff **sit_gro_receive(struct sk_buff **head,
                         ^
Just drop it for now.

Fixes: 066b300 ("tunnels: Don't apply GRO to multiple layers of encapsulation.")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>

net/ipv6/ip6_offload.c

@@ -261,19 +261,6 @@ static struct sk_buff **ipv6_gro_receive(struct sk_buff **head,
 	return pp;
 }
 
-static struct sk_buff **sit_gro_receive(struct sk_buff **head,
-					struct sk_buff *skb)
-{
-	if (NAPI_GRO_CB(skb)->encap_mark) {
-		NAPI_GRO_CB(skb)->flush = 1;
-		return NULL;
-	}
-
-	NAPI_GRO_CB(skb)->encap_mark = 1;
-
-	return ipv6_gro_receive(head, skb);
-}
-
 static int ipv6_gro_complete(struct sk_buff *skb, int nhoff)
 {
 	const struct net_offload *ops;