Allow client certificates without openssl

We don't use OpenSSL, but only depended on it because QtNetwork didn't
define QSslCertificate without an SSL backend.

I changed that in QtNetwork 5.12, so we only rely on a QtNetwork SSL
backend with older Qt versions.

Task-number: QTBUG-54877
Change-Id: I16f3077235f5c48610597868608ef2e5ca2efa99
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>