code: public boolean unregisterTaskManager(InstanceID instanceId) {
  		checkInit();
  
  		LOG.debug("Unregister TaskManager {} from the SlotManager.", instanceId);
  
  		TaskManagerRegistration taskManagerRegistration = taskManagerRegistrations.remove(instanceId);
  
  		if (null != taskManagerRegistration) {
  			internalUnregisterTaskManager(taskManagerRegistration);
  
  			return true;
  		} else {
  			LOG.debug("There is no task manager registered with instance ID {}. Ignoring this message.", instanceId);
  
  			return false;
  		}
  	}
docs: Unregisters the task manager identified by the given instance id and its associated slots
  from the slot manager.
  
  @param instanceId identifying the task manager to unregister
  @return True if there existed a registered task manager with the given instance id