remove device-is-busy property

remove device-is-busy property

This should never have been a property since

 - we are not really ready to make that kind of guarantee; another
   program may have the device open and then we're going to fail
   with EBUSY and return a Busy D-Bus error

 - it is computed on demand and that makes it difficult to determine
   when it changes value and thus trigger a PropertyChanged event

Currently this property is not used by gnome-disk-utility. If it turns
out that we need something like this we can add a Device.IsBusy()
method.