Replace SSL_PKEY_RSA_ENC, SSL_PKEY_RSA_SIGN

The original intent of SSL_PKEY_RSA_SIGN and SSL_PKEY_RSA_ENC was to
support two different keys for RSA signing and decrypt. However this
was never implemented and we only ever set one key and the other was
always NULL. Replace with single SSL_PKEY_RSA type.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2587)