Remove unnecessary check for presence of supported EC list

For both client/server the EC curve list is assumed not to be NULL:

- On the client-side, it's assumed when writing the
  supported elliptic curve extension:

    c54ee936d7/library/ssl_cli.c (L316)

- On the server, it is assumed when searching for a
  suitable curve for the ECDHE exchange:

    c54ee936d7/library/ssl_srv.c (L3200)

It is therefore not necessary to check this in mbedtls_ssl_check_curve().

library/ssl_tls.c

@@ -11241,9 +11241,6 @@ unsigned char mbedtls_ssl_hash_from_md_alg( int md )
  */
 int mbedtls_ssl_check_curve( const mbedtls_ssl_context *ssl, mbedtls_ecp_group_id grp_id )
 {
-    if( ssl->conf->curve_list == NULL )
-        return( -1 );
-
     MBEDTLS_SSL_BEGIN_FOR_EACH_SUPPORTED_EC_GRP_ID( own_ec_id )
     if( own_ec_id == grp_id )
         return( 0 );