Remove an old comment introduced in golang.org/cl/9073.

Change-Id: I14be27ddfac987f44d839920bc4d02361a576f06
Reviewed-on: https://go-review.googlesource.com/66371
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Ian Lance Taylor <iant@golang.org>