Called when the generator is destroyed, after having left the processing thread.
Here you can clean up any resources that were allocated for the control or realtime thread.
It may take an indeterminate amount of time between ControlContext.Destroy is called and this
will be called, and there may be any number of ProcessorInstance.IControl<T0>.Update calls inbetween.
This is because destroying realtimes is an asynchronous process.
Additional resources: RootOutputInstance.IRealtime.RemovedFromProcessing, ControlContext.Destroy