daemon: Fix reference counting at daemon shutdown
Commit b78a2ab9 ("daemon: Stop passing UpDevice reference to the daemon") fixed the fact that the signal passed a reference to the daemon. This fix also implied that the UpDaemon object does not own a reference to the device itself, instead relying on the UpDeviceList to keep it alive.
With this change, there is no need to drop the extra reference when clearing the device list, remove the code accordingly.