stand/userboot: Kill set but unused variables

We never use 'dev' after fetching it from the varargs list. Skip the
whole bother of fetching it, or setting up the meachinery to fetch it.

Sponsored by:		Netflix

stand/userboot/userboot/host.c

@@ -135,12 +135,6 @@ host_dev_print(int verbose)
 static int
 host_dev_open(struct open_file *f, ...)
 {
-	va_list		args;
-	struct devdesc	*dev;
-
-	va_start(args, f);
-	dev = va_arg(args, struct devdesc*);
-	va_end(args);
 
 	return (0);
 }