xwayland: Don't need xwl_window anymore in xwl_present_queue_vblank

Fixes issue #12. Presumably the problem was that Present operations on
unmapped windows were executed immediately instead of only when reaching
the target MSC.

(cherry picked from commit f541615342)

hw/xwayland/xwayland-present.c

@@ -350,13 +350,9 @@ xwl_present_queue_vblank(WindowPtr present_window,
                          uint64_t event_id,
                          uint64_t msc)
 {
-    struct xwl_window *xwl_window = xwl_window_from_window(present_window);
     struct xwl_present_window *xwl_present_window = xwl_present_window_get_priv(present_window);
     struct xwl_present_event *event;
 
-    if (!xwl_window)
-        return BadMatch;
-
     event = malloc(sizeof *event);
     if (!event)
         return BadAlloc;