cmd/compile: delete ZeroAuto

cmd/compile: delete ZeroAuto

ZeroAuto was used with the ambiguously live logic. The
ambiguously live logic is removed as we switched to stack
objects. It is now never called. Remove.

Change-Id: If4cdd7fed5297f8ab591cc392a76c80f57820856
Reviewed-on: https://go-review.googlesource.com/c/go/+/203538
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>