net-tcp: remove useless tw_timeout field

Tested: 'git grep tw_timeout' comes up empty and it builds :-)

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