Remove legacy sign/verify from EVP_MD.

Remove sign/verify and required_pkey_type fields of EVP_MD: these are a
legacy from when digests were linked to public key types. All signing is
now handled by the corresponding EVP_PKEY_METHOD.

Only allow supported digest types in RSA EVP_PKEY_METHOD: other algorithms
already block unsupported types.

Remove now obsolete EVP_dss1() and EVP_ecdsa().

Reviewed-by: Richard Levitte <levitte@openssl.org>