ssl: ignore CertificateRequest's content for real

- document why we made that choice
- remove the two TODOs about checking hash and CA
- remove the code that parsed certificate_type: it did nothing except store
  the selected type in handshake->cert_type, but that field was never accessed
afterwards. Since handshake_params is now an internal type, we can remove that
field without breaking the ABI.

include/mbedtls/ssl_internal.h

@@ -166,7 +166,6 @@ struct mbedtls_ssl_handshake_params
      * Handshake specific crypto variables
      */
     int sig_alg;                        /*!<  Hash algorithm for signature   */
-    int cert_type;                      /*!<  Requested cert type            */
     int verify_sig_alg;                 /*!<  Signature algorithm for verify */
 #if defined(MBEDTLS_DHM_C)
     mbedtls_dhm_context dhm_ctx;                /*!<  DHM key exchange        */