FCPClientReferenceImplementation: Remove obsolete class

FCPScoresSynchronizationHandler was replaced by
FCPBeginSynchronizationNotificationHandler.
FCPClientReferenceImplementation: Remove obsolete variable

There aren't any of those handlers being created anymore.
FCP{Begin/End}SynchronizationNotificationHandler: Allow callback to fail

- What triggers these handlers is a SubscriptionManager.Notification.
The SubscriptionManager is perfectly capable of letting a client fail
processing of a Notification, and to resend it afterwards. So we should
expose this functionality to the client.
- The JavaDoc is copypasted from SubscribedObjectChangedHandler and thus
does not meet the line length limit yet.