do not attempt to mount a cluster secret if none were provided.

[jmazzitelli]

also, deprecated features_flags.clustering is removed

fixes: kiali/kiali#7187

[jmazzitelli]

1. Create minikube cluster (e.g. hack/k8s-minikube.sh start) and install Istio (e.g. hack/istio/install-istio-via-istioctl.sh -c kubectl)
2. Build, push, and deploy server and operator (make -e CLUSTER_TYPE=minikube build build-ui cluster-push operator-create kiali-create)
3. Now edit the Kiali CR to set up some dummy remote clusters (kubectl edit kiali kiali -n kiali-operator). Add this to the Kiali CR (i.e., under spec create the clustering section:

spec:
  clustering:
    clusters:
    - name: accessible-cluster
      secret_name: the-accessible-cluster-secret
    - name: not-accessible-cluster

4. Now examine the volumes and volume mounts of the kiali pod; you should NOT see not-accessible-cluster but you should see accessible-cluster. (e.g. kubectl get pods -n istio-system -l app.kubernetes.io/name=kiali -oyaml)

If you have yq you can more easily see it via this:

kubectl get pods -n istio-system -l app.kubernetes.io/name=kiali -oyaml | yq e '.items[0].spec.containers[0].volumeMounts'

which results in something like this (notice accessible-cluster but no not-accessible-cluster):

- mountPath: /kiali-configuration
  name: kiali-configuration
- mountPath: /kiali-secret
  name: kiali-secret
- mountPath: /kiali-cabundle
  name: kiali-cabundle
- mountPath: /kiali-override-secrets/login-token-signing-key
  name: login-token-signing-key
  readOnly: true
- mountPath: /kiali-remote-cluster-secrets/the-accessible-cluster-secret
  name: accessible-cluster
  readOnly: true
- mountPath: /var/run/secrets/kubernetes.io/serviceaccount
  name: kube-api-access-j5qz6
  readOnly: true

Similarly with volumes:

kubectl get pods -n istio-system -l app.kubernetes.io/name=kiali -oyaml | yq e '.items[0].spec.volumes'

results in something like this:

- configMap:
    defaultMode: 420
    name: kiali
  name: kiali-configuration
- name: kiali-secret
  secret:
    defaultMode: 420
    optional: true
    secretName: kiali
- configMap:
    defaultMode: 420
    name: kiali-cabundle
    optional: true
  name: kiali-cabundle
- name: login-token-signing-key
  secret:
    defaultMode: 420
    items:
      - key: key
        path: value.txt
    optional: false
    secretName: kiali-signing-key
- name: accessible-cluster
  secret:
    defaultMode: 420
    secretName: the-accessible-cluster-secret
- name: kube-api-access-j5qz6
  projected:
    defaultMode: 420
    sources:
      - serviceAccountToken:
          expirationSeconds: 3607
          path: token
      - configMap:
          items:
            - key: ca.crt
              path: ca.crt
          name: kube-root-ca.crt
      - downwardAPI:
          items:
            - fieldRef:
                apiVersion: v1
                fieldPath: metadata.namespace
              path: namespace