cmd/compile/internal/types2: remove useConstraintTypeInference flg (cleanup)

cmd/compile/internal/types2: remove useConstraintTypeInference flg (cleanup)

This useConstraintTypeInference flag was debugging purposes only and
is not needed anymore. It's already gone in go/types.

Also, adjust/fix some comments.

Change-Id: I713be5759f05c618fcf26e16cf53dfb3626bba93
Reviewed-on: https://go-review.googlesource.com/c/go/+/461690
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>