tcp: remove SG-related comment in tcp_sendmsg()

Since commit 74d4a8f8 ("tcp: remove sk_can_gso() use"), the code
doesn't care whether the interface supports SG.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>