Remove dead getrusage replacement code.

Remove dead getrusage replacement code.

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

Note that POSIX only covers ru_utime and ru_stime and we rely on many
more fields without any kind of configure probe, but that predates this
commit.

The only supported system we need replacement code for now is Windows,
and that can be done without a configure probe.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Greg Stark <stark@mit.edu>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/CA+hUKGJ3LHeP9w5Fgzdr4G8AnEtJ=z=p6hGDEm4qYGEUX5B6fQ@mail.gmail.com