redhat_subscription: manually unregister only when registered (#6259)
When registering using D-Bus and using a version of subscription-manager
with an unimplemented 'force' option, then unregister manually the
system only if it is registered. 'subscription-manager unregister'
errors out when trying to unregister an already unregistered system.
(cherry picked from commit c9e11e5c0c)
Co-authored-by: Pino Toscano <ptoscano@redhat.com>
