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>