Remove configure probe and related tests for getrlimit.

Remove configure probe and related tests for getrlimit.

getrlimit() is in SUSv2 and all targeted systems have it.

Windows doesn't have it.  We could just use #ifndef WIN32, but for a
little more explanation about why we're making things conditional, let's
retain the HAVE_GETRLIMIT macro.  It's defined in port.h for Unix systems.

On systems that have it, it's not necessary to test for RLIMIT_CORE,
RLIMIT_STACK or RLIMIT_NOFILE macros, since SUSv2 requires those and all
targeted systems have them.  Also remove references to a pre-historic
alternative spelling of RLIMIT_NOFILE, and coding that seemed to believe
that Cygwin didn't have it.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA+hUKGJ3LHeP9w5Fgzdr4G8AnEtJ=z=p6hGDEm4qYGEUX5B6fQ@mail.gmail.com