'Remove the host of host_id from the model. Return None.
@preconditions: host_id must be in self.host_id_to_iter,
self.host_id_to_iter[host_id] must be in model'

def _remove_single_host_from_model(self, host_id):

    host_iter = self.host_id_to_iter[host_id]
    del self.host_id_to_iter[host_id]
    self.model.remove(host_iter)
    self.host_amount_total -= 1
    self.host_amount_in_model -= 1
