Cleanup mttest.c : because we no longer use stdio here, don't include it

Reviewed-by: NRich Salz <rsalz@openssl.org>

crypto/threads/mttest.c

@@ -56,7 +56,6 @@
  * [including the GNU Public Licence.]
  */
 
-#include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <errno.h>