dns: simplify dns.promises warning logic

dns.promises is lazy loaded. Instead of using a seaparate
Boolean flag to track whether or not it has been loaded, just
inspect the state of the lazy loaded module itself.

PR-URL: https://github.com/nodejs/node/pull/24788


Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Masashi Hirano <shisama07@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>