Remove configure probe for sys/uio.h.

Remove configure probe for sys/uio.h.

<sys/uio.h> is in SUSv2, and all targeted Unix system have it, so we
might as well drop the probe (in fact we never really needed this one).
It's where struct iovec is defined, and as a common extension, it's also
where non-standard preadv() and pwritev() are declared on systems that
have them.

We should also be able to assume that IOV_MAX is defined on Unix.

To spell out what our pg_iovec.h header does for the OSes in the build
farm as of today:

  Windows: our own struct and functions
  Solaris, Cygwin: <sys/uio.h>'s struct, our own functions
  Every other Unix: <sys/uio.h>'s struct and functions

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CA%2BhUKG%2BL_3brvh%3D8e0BW_VfX9h7MtwgN%3DnFHP5o7X2oZucY9dg%40mail.gmail.com