cmd/compile: remove a few unused bits of code

Just the low-hanging fruit; code that was either never used, or its last
use was removed a while ago.

Change-Id: Ic7f90aeee3e6daf7edc16cde3c0a767d67e617ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/192618


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>