cmd/compile/internal/importer: turn off debugging output

cmd/compile/internal/importer: turn off debugging output

Also, remove `debug` constant. Was not used.
Follow-up on CL 442303.

Fixes #56681.

Change-Id: Ia1499511ba553670617bcb9b7c699412e8df0669
Reviewed-on: https://go-review.googlesource.com/c/go/+/449238
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>