[ruby/openssl] pkey: remove unnecessary prototype from ossl_pkey.h

ossl_ec_new() was removed in commit ruby/openssl@94aeab2f265d (pkey: simplify
ossl_pkey_new(), 2017-03-16), but it forgot to remove the declaration
while doing so.

ruby/openssl@faad7a0811

Author: rhenium

ext/openssl/ossl_pkey.h

@@ -71,7 +71,6 @@ void Init_ossl_dh(void);
  * EC
  */
 extern VALUE cEC;
-VALUE ossl_ec_new(EVP_PKEY *);
 void Init_ossl_ec(void);
 
 #define OSSL_PKEY_BN_DEF_GETTER0(_keytype, _type, _name, _get)          \