window: Fix meta_window_set_alive() not working if first ping fails

window->is_alive isn't initialized explicitly so it defaults to FALSE
meaning that if the first ping fails we'd short circuit and not show
the delete dialog as we should.

We could initialize the variable to TRUE but in fact we don't even
need the variable at all since our dialog management is enough to
manage all the state we need, i.e. we're only interested in knowing
whether we're already displaying a delete dialog.

This does change our behavior here since previously we wouldn't
display the dialog again if the next ping failed after the dialog is
dismissed but this was arguably a bug too since in that case there
wouldn't be a way to kill the window after waiting for a while and the
window kept being unresponsive.

https://bugzilla.gnome.org/show_bug.cgi?id=749711

src/core/delete.c

@@ -160,12 +160,7 @@ void
 meta_window_set_alive (MetaWindow *window,
                        gboolean    is_alive)
 {
-  if (window->is_alive == is_alive)
-    return;
-
-  window->is_alive = is_alive;
-
-  if (window->is_alive)
+  if (is_alive)
     kill_delete_dialog (window);
   else
     show_delete_dialog (window, CurrentTime);

src/core/window-private.h

@@ -434,7 +434,6 @@ struct _MetaWindow
 
   /* Managed by delete.c */
   int dialog_pid;
-  guint is_alive : 1;
 
   /* maintained by group.c */
   MetaGroup *group;