[2.5, UI] Removing a 'dnsresource' over the UI doesn't seem to remove it from the database

[Andres Rodriguez]

I created a DNS resource over the API, I then deleted it over the UI, and try to created over the API again, and it failed, because it was still in the database.

However, if I delete the DNS resource over the API, I can create a new one. To reproduce:

1. Create a resource maas admin dnsresources create name=test ip_addresses=192.168.122.1 domain=maas
2. Delete over the UI
3. Attempt to create it again and it failed:
root@maas-bionic:~# maas admin dnsresources create name=test ip_addresses=192.168.122.1 domain=maas
{"__all__": ["Labels must be unique within their zone."]}
4. Checking resources over the API, still shows it there:

root@maas-bionic:~# maas admin dnsresources read
Success.
Machine-readable output follows:
[
    {
        "address_ttl": null,
        "ip_addresses": [],
        "fqdn": "test.maas",
        "id": 3,
        "resource_records": [],
        "resource_uri": "/MAAS/api/2.0/dnsresources/3/"
    }
]