Unregistering a system also drops all the resources for it
automatically, so there is no need to manually unsubscribing (which
actually means removing all the subscriptions).
In addition to that, newer versions of subscription-manager drop all the
support for entitlements, so the "remove" subcommand (used by
unsubscribe()) does not exist anymore, and thus the unregistration fails
with those versions.
This fixes the registration on EL 10 systems, and Fedora 41 and greater.
