Revert "sit: Add gro callbacks to sit_offload"

This patch reverts 19424e05 ("sit:
Add gro callbacks to sit_offload") because it generates packets
that cannot be handled even by our own GSO.
Reported-by: Wolfgang Walter <linux@stwm.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_offload.c

@@ -292,8 +292,6 @@ static struct packet_offload ipv6_packet_offload __read_mostly = {
 static const struct net_offload sit_offload = {
 	.callbacks = {
 		.gso_segment	= ipv6_gso_segment,
-		.gro_receive	= ipv6_gro_receive,
-		.gro_complete	= ipv6_gro_complete,
 	},
 };