Simplify some code

The original curve448 code was templated to allow for a 25519
implementation. We've just imported the 448 stuff - but a remnant of
the original templated approach remained. This just simplifies that.

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/5105)