Remove configure probe for netinet/tcp.h.

<netinet/tcp.h> is in SUSv3 and all targeted Unix systems have it.
For Windows, we can provide a stub include file, to avoid some #ifdef
noise.

Discussion: https://postgr.es/m/CA+hUKGKErNfhmvb_H0UprEmp4LPzGN06yR2_0tYikjzB-2ECMw@mail.gmail.com

src/backend/libpq/be-secure-openssl.c

@@ -24,10 +24,8 @@
 #include <unistd.h>
 #include <netdb.h>
 #include <netinet/in.h>
-#ifdef HAVE_NETINET_TCP_H
 #include <netinet/tcp.h>
 #include <arpa/inet.h>
-#endif
 
 #include "libpq/libpq.h"
 #include "miscadmin.h"

src/backend/libpq/be-secure.c

@@ -24,10 +24,8 @@
 #include <sys/socket.h>
 #include <netdb.h>
 #include <netinet/in.h>
-#ifdef HAVE_NETINET_TCP_H
 #include <netinet/tcp.h>
 #include <arpa/inet.h>
-#endif
 
 #include "libpq/libpq.h"
 #include "miscadmin.h"

src/backend/libpq/ifaddr.c

@@ -24,9 +24,7 @@
 #include <sys/socket.h>
 #include <netdb.h>
 #include <netinet/in.h>
-#ifdef HAVE_NETINET_TCP_H
 #include <netinet/tcp.h>
-#endif
 #include <sys/file.h>
 
 #include "libpq/ifaddr.h"

src/backend/libpq/pqcomm.c

@@ -67,9 +67,7 @@
 #include <sys/time.h>
 #include <netdb.h>
 #include <netinet/in.h>
-#ifdef HAVE_NETINET_TCP_H
 #include <netinet/tcp.h>
-#endif
 #include <utime.h>
 #ifdef _MSC_VER					/* mstcpip.h is missing on mingw */
 #include <mstcpip.h>