It's not used, it's never been used, and it doesn't do what its doc
comment says it does.

Fixes #18941.

Change-Id: Ia89d97fb87525f5b861d7701f919e0d6b7cbd376
Reviewed-on: https://go-review.googlesource.com/36322Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>