cmd/go: remove temporary rf cleanup script

This commit is part of the overall effort to eliminate global
modloader state.

Change-Id: I9d2deebafbbb374de2a1f9bae99e9caf417313a0
Reviewed-on: https://go-review.googlesource.com/c/go/+/709991
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Michael Matloob <matloob@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: jitsu-net