tcp: Remove dead sysctl_tcp_cookie_size declaration

Remove a declaration left over from the TCPCT-ectomy. This sysctl is
no longer referenced anywhere since 1a2c6181 ("tcp: Remove TCPCT").

Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>