lookupName is only called in one location, and one of the return
values is unused, so let's remove it.

Change-Id: I35e22c7ec611e8eb349deb4f0561e212f7d9de0b
Reviewed-on: https://go-review.googlesource.com/55232Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>