cmd/compile/internal/types2: remove concept of finals

This is a 1:1 port of the respective change in go/types
in https://golang.org/cl/299590.

Change-Id: I65ad723f2e21e3d95fc0b94665e0121e31871a48
Reviewed-on: https://go-review.googlesource.com/c/go/+/300250


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>