Remove the curve448 vector code

We removed various platform specific optimisation files in an earlier
commit. The vector code was related to that and therefore is no longer
required. It may be resurrected at a later point if we reintroduce the
opimtisations.

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