misc: delete benchcmp forwarding script

benchcmp was moved out of misc into x/tools in CL 60100043 in 2014,
and then replaced by a forwarding script in CL 82710043.
Five years have since passed, and the forwarding script has outlived
its usefulness. It's now more confusing than helpful. Delete it.

Change-Id: I8c7d65b97e0b3fe367df69a86ae10c7960c05be3
Reviewed-on: https://go-review.googlesource.com/c/go/+/202762Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

misc/benchcmp

-#!/bin/bash
-
-echo 'misc/benchcmp has moved:' >&2
-echo '	go get -u golang.org/x/tools/cmd/benchcmp' >&2
-exit 2