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

This reverts commit fda8ee8b.

Reason for revert: Interface.Complete is still necessary for safe concurrency.

For #47726

Change-Id: I8b924ca5f4af8c7d7e2b5a27bb03a5a5ed9b1d22
Reviewed-on: https://go-review.googlesource.com/c/go/+/342710


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>