Remove configure probe for sys/sockio.h.

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.