Remove configure probe for sys/sockio.h.

On BSD-family systems, header <sys/sockio.h> defines socket ioctl
numbers like SIOCGIFCONF.  Only AIX is using those now, but it defines
them in <net/if.h> anyway.

Supposing some PostgreSQL hacker wants to test that AIX-only code path
on a more common development system by pretending not to have
getifaddrs().  It's enough to include <sys/ioctl.h>, at least on macOS,
FreeBSD and Linux, and we're already doing that.

src/backend/libpq/ifaddr.c

@@ -325,10 +325,6 @@ pg_foreach_ifaddr(PgIfAddrCallback callback, void *cb_data)
 #include <sys/ioctl.h>
 #include <net/if.h>
 
-#ifdef HAVE_SYS_SOCKIO_H
-#include <sys/sockio.h>
-#endif
-
 #if defined(SIOCGIFCONF)
 
 /*