go/types, types2: remove Qualifier parameter from Checker.implements

Where we provide it we take it from the Checker (which is already
passed in). Thus there's no need to pass it separately. Cleanup.

Change-Id: I63ae445ccac5643235d85e1867462ef5c01ad5fe
Reviewed-on: https://go-review.googlesource.com/c/go/+/381297


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>