autoboot: Drop unused CONFIG_MENUPROMPT

This is not defined by any board. We could use CONFIG_AUTOBOOT_PROMPT
instead perhaps, but this depends on CONFIG_AUTOBOOT_KEYED which is not
used for the single-key case.

So let's just remove CONFIG_MENUPROMPT.

Signed-off-by: Simon Glass <sjg@chromium.org>

common/autoboot.c

@@ -233,11 +233,7 @@ static int abortboot_single_key(int bootdelay)
 	int abort = 0;
 	unsigned long ts;
 
-#ifdef CONFIG_MENUPROMPT
-	printf(CONFIG_MENUPROMPT);
-#else
 	printf("Hit any key to stop autoboot: %2d ", bootdelay);
-#endif
 
 	/*
 	 * Check if key already pressed