cmd/internal/obj/s390x: cleanup objz.go

cmd/internal/obj/s390x: cleanup objz.go

This CL deletes some unnecessary code in objz.go that existed to
support instruction scheduling. It's likely instruction scheduling
will never be done in this part of the backend so this code can
just be deleted.

This file can probably be cleaned up a bit more, but I think this
is a good start.

Passes: go build -toolexec 'toolstash -cmp' -a std.

Change-Id: I1645632ac551a90a4f4be418045c046b488e9469
Reviewed-on: https://go-review.googlesource.com/38394
Run-TryBot: Michael Munday <munday@ca.ibm.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>