crypto/x509: rollback new CertificateRequest fields

In general, we don't want to encourage reading them from CSRs, and
applications that really want to can parse the Extensions field.

Note that this also fixes a bug where the error of
parseKeyUsageExtension was not handled in parseCertificateRequest.

Fixes #43477
Updates #37172

Change-Id: Ia5707b0e23cecc0aed57e419a1ca25e26eea6bbe
Reviewed-on: https://go-review.googlesource.com/c/go/+/281235


Trust: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>