[dev.typeparams] cmd/compile/internal/types2: remove Interface.Complete (cleanup)

Interface.Complete is not needed anymore. We can remove it in
types2 (and eventually make it an empty function in go/types,
where we must maintain the existing API).

Change-Id: I689f0d6f3a83997d8ca5bae773b9af0083d0bf4f
Reviewed-on: https://go-review.googlesource.com/c/go/+/340255


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>