src: check curve ID existence instead of asn flags

Simplify the code. The flag check was in the OpenSSL source, but reading
through the docs and source, it is not necessary.

Refs: https://github.com/nodejs/node/pull/24358/files#r243099693
PR-URL: https://github.com/nodejs/node/pull/25345


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>