Make legacy node health check (rabbitmq-diagnostics node_health_check) a no-op

[michaelklishin]

rabbitmq-diagnostics node_health_check (née rabbitmqctl node_health_check) has been deprecated for well over a year. It is a very opinionated, very expensive health check that is often abused and performed unreasonably often, which leads to excessive system load.

By making it a no-op we will side step such (fairly common) operator mistakes and further promote the new set of fine-grained health checks.