src: remove cast for unsupported openssl

The cast is needed to build against OpenSSL 1.0.2, which master, 11.x,
and 10.x no longer support.

PR-URL: https://github.com/nodejs/node/pull/26305


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>