tcpbench: cleanup headers

With the use of TCP_INFO and struct tcp_info none of those kernel internal
headers are needed anymore.

Code still compiles for me.
-- 
:wq Claudio

Index: tcpbench.c
===================================================================
RCS file: /cvs/src/usr.bin/tcpbench/tcpbench.c,v
diff -u -p -r1.76 tcpbench.c
--- tcpbench.c	6 Dec 2025 09:48:30 -0000	1.76
+++ tcpbench.c	27 Jul 2026 12:43:15 -0000
@@ -30,10 +30,6 @@
 #include <netinet/in.h>
 #include <netinet/ip.h>
 #include <netinet/tcp.h>
-#include <netinet/tcp_timer.h>
-#include <netinet/tcp_fsm.h>
-#include <netinet/in_pcb.h>
-#include <netinet/tcp_var.h>
 
 #include <arpa/inet.h>