tcp: tcp_sendmsg() only deals with CHECKSUM_PARTIAL

We no longer have skbs with skb->ip_summed == CHECKSUM_NONE
in TCP write queues.

We can remove dead code in tcp_sendmsg().

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>