Simplify X509_REQ_extension_nid()

Simplify X509_REQ_extension_nid()

Now that the global ext_nids[] array can no longer be modified by the
application, we can simplify this by returning the two possible NIDs
that we accept in the extension list attribute in PKCS#10 certification
requests.

The year is 2024. This API is entirely unused by the ecosystem. Well not
entirely! One small village of indomitable rare API use still holds out
against the cleansers. You may have guessed it: security/xca.

ok jsing