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

TestIncompleteInterfaces is not useful anymore because interface
printing always shows the syntactic type structure of an interface.

Also remove the respective support code in interface printing and
simplify that code.

Move the newDefined and nopos support declarations unchanged into
api_test.go where they are used.

Updates #46167.

Change-Id: I23e303bc4ae4271912ba75f201bd2b7cd4a17b3e
Reviewed-on: https://go-review.googlesource.com/c/go/+/339832


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