cmd/compile/internal/types2: remove 'strict' argument from several methods

The value is always 'false'. Brings the code closer in line with go/types.
Follow-up on https://golang.org/cl/304129.

Change-Id: I8bea550033f3187b44e9a54258e0cf642c11c369
Reviewed-on: https://go-review.googlesource.com/c/go/+/304849


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