crypto: purge 'loaded' property that was not fully removed

crypto: purge 'loaded' property that was not fully removed

The 'loaded' property on TLS creds and secret objects was marked as
deprecated in 6.0.0. In 7.1.0 the deprecation info was moved into
the 'removed-features.rst' file, but the property was not actually
removed, just made read-only. This was a highly unusual practice,
so finish the long overdue removal job.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>