net: skb_needs_check() removes CHECKSUM_UNNECESSARY check for tx.

Because we remove the UFO support, we will also remove the
CHECKSUM_UNNECESSARY check in skb_needs_check().

Cc: Willem de Bruijn <willemb@google.com>
Signed-off-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>