runtime: remove unused getrlimit on linux/riscv64

runtime: remove unused getrlimit on linux/riscv64

Follow CL 94775 and CL 93655 which removed the (commented-out) usage
of this function on other platforms.

Change-Id: I28e0569d8531d0c09f3caefa7c4eb54fb5bd8a33
Reviewed-on: https://go-review.googlesource.com/c/go/+/334429
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>