crypto: remove DiffieHellman.initialised_

As pointed out by Ben Noordhuis, this internal field can be removed
since all instances are initialized when exposed to users.

PR-URL: https://github.com/nodejs/node/pull/23717
Refs: https://github.com/nodejs/node/pull/23648


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>