You are looking at documentation for an alpha release. Not what you want? See Deploy Elasticsearch & Kibana on Kubernetes documentation.
Before uninstalling the operator, remove all Elastic resources:
kubectl delete elastic --all --all-namesapces
This deletes all underlying Elasticsearch, Kibana, and APM Server resources (pods, secrets, services, etc.).
Then, you can uninstall the operator:
kubectl delete -f https://download.elastic.co/downloads/eck/0.9.0/all-in-one.yaml
And remove the webhook configuration:
kubectl delete validatingwebhookconfigurations validating-webhook-configuration