syscall: remove TestRlimit

It's more trouble than it's worth. New code should be using x/sys/unix
anyhow.

Fixes #40564
Fixes #51479

Change-Id: I1c0e13f494380c1565e98359f088af9f52790b79
Reviewed-on: https://go-review.googlesource.com/c/go/+/390020


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>