all: remove some unused return parameters

all: remove some unused return parameters

As found by unparam. Picked the low-hanging fruit, consisting only of
errors that were always nil and results that were never used. Left out
those that were useful for consistency with other func signatures.

Change-Id: I06b52bbd3541f8a5d66659c909bd93cb3e172018
Reviewed-on: https://go-review.googlesource.com/102418
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>