Remove unused manifest (docs) (#12092)

This file is no longer referenced since e0d67367 (Update installation
doc with vagrant (#8406), 2022-01-11).

contrib/misc/clusteradmin-rbac.yml

----
-apiVersion: rbac.authorization.k8s.io/v1
-kind: ClusterRoleBinding
-metadata:
-  name: kubernetes-dashboard
-  labels:
-    k8s-app: kubernetes-dashboard
-roleRef:
-  apiGroup: rbac.authorization.k8s.io
-  kind: ClusterRole
-  name: cluster-admin
-subjects:
-- kind: ServiceAccount
-  name: kubernetes-dashboard
-  namespace: kube-system