Charm should remove inappropriate checks if not needed

[Paul Goins]

The infra-node charm appears to add appropriate MAAS checks, but does nothing to remove inappropriate ones.

Consider the code here:

-----
        if self.is_maas_snap_loaded():
            services = ['snap.maas.supervisor']
        else:
            services = ['maas-proxy', 'maas-rackd', 'maas-regiond', 'bind9']
            if config.get('nagios_check_dhcpd'):
                services.append('maas-dhcpd')
        nrpe.add_init_service_checks(nrpe_setup, services, current_unit)
        nrpe_setup.write()
-----

If the environment were using deb-based MAAS and switched to using snap-based MAAS - while the new checks would be added, nothing would be done to remove the old checks.

We appear to have reproduced this bug on a customer cloud, where we concretely have checks for snap.maas.supervisor in addition to maas-regiond, maas-rackd, etc.