I don't see how the call could fail, so, no test. Just a code cleanup in
case it can fail in the future.

Fixes #15987.

Change-Id: If4af0d5e7d19cc8b13fb6a4f7661c37fb0015e83
Reviewed-on: https://go-review.googlesource.com/23860
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>