Revert "sit: Add gro callbacks to sit_offload"

This patch reverts 19424e052fb44da2f00d1a868cbb51f3e9f4bbb5 ("sit:
Add gro callbacks to sit_offload") because it generates packets
that cannot be handled even by our own GSO.

Reported-by: Wolfgang Walter <[e-mail]>
Signed-off-by: Herbert Xu <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv6/ip6_offload.c

···
 static const struct net_offload sit_offload = {
 	.callbacks = {
 		.gso_segment	= ipv6_gso_segment,
-		.gro_receive	= ipv6_gro_receive,
-		.gro_complete	= ipv6_gro_complete,
 	},
 };