Delete Clients in KsysGuardDaemon destructor

Clients and plugins are both children of the daemon. The Client destructor dereferences SensorProperties to unsubscribe from them. Because usually the object tree looks something like plugin -> sensorcontainer -> sensorobject -> sensorproperty we can not rely on the properties still being around when the clients are deleted. This fixes the segfault in the unit test detected by the address sanitizer.