Revert "Bad cast from SanExtensionImpl to CertificateExtension_XmlSecImpl"

This reverts commit 321b8ff8.

Revert "loplugin:singlevalfields in writerfilter and xmlsec"

This reverts commit a25327e4.

cause these two together mean that we get a certificate is
invalid dialog when the cert is perfectly fine on loading https
sources

Change-Id: Id8c1fb5ece4ecc0035500ce0b822a6b248d14282