cmd: remove unused code

cmd: remove unused code

Generated with honnef.co/go/unused

There is a large amount of unused code in cmd/internal/obj/s390x but
that can wait til the s390x port is merged.

There is some unused code in
cmd/internal/unvendor/golang.org/x/arch/arm/armasm but that should be
addressed upstream and a new revision imported.

Change-Id: I252c0f9ea8c5bb1a0b530a374ef13a0a20ea56aa
Reviewed-on: https://go-review.googlesource.com/21782
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Dave Cheney <dave@cheney.net>