cmd/internal/gc, etc: remove dead code

cmd/internal/gc, etc: remove dead code

Found with https://github.com/opennota/check.

Change-Id: I50c173382782fb16b15100e02c1c85610bc233a0
Reviewed-on: https://go-review.googlesource.com/7130
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>