cmd/compile/internal/types2: remove Config.InferFromConstraints flag

Constraint type inference is part of the proposed language.
Use an internal flag to control the feayure for debugging.

Change-Id: I7a9eaee92b5ffc23c25d9e68a729acc0d705e879
Reviewed-on: https://go-review.googlesource.com/c/go/+/306770


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