Remove an observer.

def deleteObserver(self, observer):
        """Remove an observer."""
        Observable.deleteObserver(self, observer)
        # If self.startOnDemand is True, the reader monitoring
        # thread is stopped when there are no more observers.
        if self.startOnDemand:
            if 0 == self.countObservers():
                self.rmthread.stop()
                del self.rmthread
                self.rmthread = None