Fix gettable_params() for ECX

Fix gettable_params() for ECX

The OSSL_PKEY_PARAM_MANDATORY_DIGEST parameter is only handled by the
ed25519_get_params() and ed448_get_params(). The x25519 and x448
versions of get_params() always ignore that parameter, so it should not
be in the list of gettable params.

Fixes: 1a7328c88256 ("PROV: Ensure that ED25519 & ED448 keys have a mandatory digest")
cla: trivial

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/27043)