crypto/x509: bypass signature verification in CreateCertificate when using MD5WithRSA

Bypasses the signature verification check we previously added if the
signature algorithm is MD5WithRSA, as we only support this algorithm
for signing and not verification.

Change-Id: Idba6dbba8b365d6199d467526746b88a5f734af1
Reviewed-on: https://go-review.googlesource.com/c/go/+/264019


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