go/types: make Interface.Complete a no-op

This is a partial port of CL 340255 to go/types. Of course we can't
delete Interface.Complete, but make it a no-op.

Completing interfaces is no longer necessary.

Change-Id: Ida3c84cc94713f14a646c7682f5d4ae5339a0faa
Reviewed-on: https://go-review.googlesource.com/c/go/+/342489


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