Change requirements for setting timer callback

The code wants timer callbacks to be set (checked in fetch_input()), and can't
easily check whether we're using nbio, so it seems easier to require the
callbacks to be always set rather than only with nbio as was previously done.

programs/ssl/ssl_server2.c

@@ -4028,7 +4028,6 @@ data_exchange:
                             opt.nbio == 0 ? mbedtls_net_recv_timeout : NULL );
 
 #if defined(MBEDTLS_TIMING_C)
-            if( opt.nbio != 0 && opt.read_timeout != 0 )
                 mbedtls_ssl_set_timer_cb( &ssl, &timer,
                                           mbedtls_timing_set_delay,
                                           mbedtls_timing_get_delay );