cmd/compile: remove special-casing of blank in types.sconv{,2}

I'm not sure why blank was special-cased here before, but it's
wrong. Blank is a non-exported identifier, and writing it out without
package-qualification can result in linker symbol collisions.

Fixes #47087.

Change-Id: Ie600037c8e54e3d4fdaeec21e2ca212badbd830b
Reviewed-on: https://go-review.googlesource.com/c/go/+/333163


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>