xen/dev: remove write-only variable

This was found while looking for driver_filter_t functions which got the
trap frame from the argument.  This particular instance it isn't even
used, so remove now lest someone else get to it first.

Reviewed by:	mhorne

sys/dev/xen/debug/debug.c

@@ -69,13 +69,11 @@ extern void
 stack_capture(struct stack *st, register_t rbp);
 
 static int
-xendebug_filter(void *arg)
+xendebug_filter(void *arg __unused)
 {
 #if defined(STACK) && defined(DDB)
 	struct stack st;
-	struct trapframe *frame;
 
-	frame = arg;
 	stack_zero(&st);
 	stack_save(&st);