prombind9exporter does not unregister all collectors

Currently it registers:

	pbe.Registry.MustRegister(version.NewCollector("bind_exporter"))
	pbe.Registry.MustRegister(pbe)
	if bind9Pid > 0 {
		procExporter := prometheus.NewProcessCollector(
			prometheus.ProcessCollectorOpts{
				PidFn: func() (int, error) {
					return int(bind9Pid), nil
				},
				Namespace: namespace,
			})
		pbe.Registry.MustRegister(procExporter)
	}

but only deregisters:

	// unregister bind9 counters from prometheus framework
	pbe.Registry.Unregister(pbe)