tcp: tcp_send_challenge_ack delete useless param `skb`

After this parameter is passed in, there is no usage, and deleting it will
 not bring any impact.

Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Benjamin Yim <yan2228598786@gmail.com>
Link: https://lore.kernel.org/r/20220109130824.2776-1-yan2228598786@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>