Revert "Support EVP_PKEY_sign() and EVP_PKEY_verify() for EdDSA"

This reverts commit a6f5b116

.

The EVP_PKEY_sign() function is intended for pre-hashed input which is
not supported by our EdDSA implementation.

See the discussion in PR 5880

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6284)