core: drop implicit support of PrivateUsers=off

core: drop implicit support of PrivateUsers=off

Follow-up for fa693fdc7e17618958c505af4b2f39ecd1c3363e.

The documentation says the option takes a boolean or one of the "self"
and "identity". But the parser uses private_users_from_string() which
also accepts "off". Let's drop the implicit support of "off".