cmd: remove a few unused parameters

They all seem pretty low-risk, and the overall diff is small.

While at it, remove one in go/build too.

Change-Id: I31df52c1c97d843b06f6c1dc63462d390db4470d
Reviewed-on: https://go-review.googlesource.com/c/go/+/203607


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>