[dev.regabi] cmd/compile: remove ONEWOBJ

After CL 283233, SSA can now handle new(typ) without the frontend to
generate the type address, so we can remove ONEWOBJ in favor of ONEW
only.

This is also not save for toolstash, the same reason with CL 284115.

Change-Id: Ie03ea36b3b6f95fc7ce080376c6f7afc402d51a3
Reviewed-on: https://go-review.googlesource.com/c/go/+/284117


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>