When some SensorClients are destroyed, eg. when KSysguard is closed,
they fail to deregister themselves from SensorAgent. This may cause a
crash when further messages arrive for them. This patch fixes that.
Move the client deregistering code from SensorBrowserModel to HostInfo
Also remove the clear function. After investigating further, I've
found out it suffers from the same problem, that it doesn't deregister
from the hosts it removes. On top of that, it leaves dangling pointers
in mSensorInfoMap. It doesn't really clear all the other internal
structures. And it is never used. So I think it's best to delete it.