Affinity constraints should not render a stray `nodeAffinity` spec section

[Leon]

When I deploy multiple units of an app with anti-affinity constraints per [1],

```bash
juju deploy --trust prometheus-k8s prom --num-units=2 \
  --constraints="tags=anti-pod.app.kubernetes.io/name=prom,anti-pod.topology-key=kubernetes.io/hostname"
```

Then the resulting statefulset has a stray "nodeAffinity" section that must be manually removed, otherwise the units are stuck in `waiting/allocating`:

```diff
-nodeAffinity:
- requiredDuringSchedulingIgnoredDuringExecution:
- nodeSelectorTerms:
- - {}
podAntiAffinity:
  requiredDuringSchedulingIgnoredDuringExecution:
    - labelSelector:
        matchExpressions:
          - key: app.kubernetes.io/name
            operator: In
            values:
              - prom
      topologyKey: kubernetes.io/hostname
```

[1]: https://discourse.charmhub.io/t/pod-priority-and-affinity-in-juju-charms/4091

[Leon (sed-i)]

Juju 3.4.2-genericlinux-amd64
MicroK8s v1.29.2 revision 6529
Ubuntu 22.04.4 on Multipass

[Ian Booth (wallyworld)]

https://github.com/juju/juju/pull/17239

[Leon (sed-i)]

❤️