Remove stage 6 check from examples
Remove stage 6 check from examples

`rabbitmqctl node_health_check` has been deprecated and removed
from management plugin docs [as of 3.8.10] because of how
expensive and prone to false positives that check is.

[Per-object] monitoring data-based alerts is a much better way to such extensive
checking.

rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website