It's not safe (it crashes), and it's also useless: if you run
multiple benchmarks in parallel you will not get reliable
timing results from any of them.

Fixes #18603.

Change-Id: I00e5a72f7c98151543cf7d5573c38383276e391a
Reviewed-on: https://go-review.googlesource.com/80841Reviewed-by: Ian Lance Taylor <iant@golang.org>