all: remove some unused result params

all: remove some unused result params

Most of these are return values that were part of a receiving parameter,
so they're still accessible.

A few others are not, but those have never had a use.

Found with github.com/mvdan/unparam, after Kevin Burke's suggestion that
the tool should also warn about unused result parameters.

Change-Id: Id8b5ed89912a99db22027703a88bd94d0b292b8b
Reviewed-on: https://go-review.googlesource.com/55910
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>