net/tcp_fastopen: remove obsolete extern

After cited commit, sysctl_tcp_fastopen_blackhole_timeout is no longer
a global variable.

Fixes: 3733be14

 ("ipv4: Namespaceify tcp_fastopen_blackhole_timeout knob")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Acked-by: Wei Wang <weiwan@google.com>
Link: https://lore.kernel.org/r/20210719092028.3016745-1-eric.dumazet@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>