overdue: fix bug where add-ons are cancelled multiple times

Overdue was cancelling add-ons but entitlement is doing the same. Since
we don't have locking in entitlement yet, we were cancelling these
twice.

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>
Merge branch 'new-paymentpluginapi' of github.com:killbill/killbill into new-paymentpluginapi