tcp: Remove unused tcpct declarations and comments

Remove declaration, 4 defines and confusing comment that are no longer used
since 1a2c6181

 ("tcp: Remove TCPCT").

Signed-off-by: Dmitry Popov <dp@highloadlab.com>
Acked-by: Christoph Paasch <christoph.paasch@uclouvain.be>
Signed-off-by: David S. Miller <davem@davemloft.net>