testpart: remove _MPRINTF_REPLACE

Support for _MPRINTF_REPLACE in mprintf.h was removed in
55452ebdff, replaced with curl_printf.h.

tests/server/testpart.c

@@ -23,8 +23,7 @@
 
 #include "getpart.h"
 
-#define _MPRINTF_REPLACE /* use our functions only */
-#include <curl/mprintf.h>
+#include "curl_printf.h"
 
 /* include memdebug.h last */
 #include "memdebug.h"