[dev.link] cmd/internal/goobj2: remove old-style types

They are no longer needed.

Also rewrite the test, as the old one no longer meaningful.

Change-Id: Id39ad6bb2a334cb6d61aa0a7c52837e0c3d62432
Reviewed-on: https://go-review.googlesource.com/c/go/+/227641


Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>