Remove unused restartable ECC state for SrvKeyExchange

Previously, ssl_ecrs_ske_start_processing was used to indicate that
the ServerKeyExchange has been fetched from the record layer, but
that parsing its ECDHE parameter component has been preempted by the
restartable ECP feature. On re-entry of ssl_parse_server_key_exchange()
in this state, the code would directly jump into the parsing routine.
However, the only non-reentrant code that's jumped over this way is
the record fetching routine mbedtls_ssl_parse_record(), which is now
made re-entrant by setting `ssl->keep_current_message = 1` in case of
pre-emption due to restartable ECC.

The ssl_ecrs_ske_start_processing state is therefore redundant and
can be removed, which is what this commit does.

include/mbedtls/ssl_internal.h

@@ -404,7 +404,6 @@ struct mbedtls_ssl_handshake_params
     enum { /* this complements ssl->state with info on intra-state operations */
         ssl_ecrs_none = 0,              /*!< nothing going on (yet)         */
         ssl_ecrs_crt_verify,            /*!< Certificate: crt_verify()      */
-        ssl_ecrs_ske_start_processing,  /*!< ServerKeyExchange: pk_verify() */
         ssl_ecrs_cke_ecdh_calc_secret,  /*!< ClientKeyExchange: ECDH step 2 */
         ssl_ecrs_crt_vrfy_sign,         /*!< CertificateVerify: pk_sign()   */
     } ecrs_state;                       /*!< current (or last) operation    */