dgram: hide _healthCheck() and _stopReceiving()

These methods are private APIs of dgram sockets, but do not
need to be exposed via the Socket prototype.

PR-URL: https://github.com/nodejs/node/pull/21923


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Wyatt Preul <wpreul@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>