subscriber-view: remove is_child_multicell() vfunc

After we added GcalEvent::is_multiday(), there is no need to
wrap it in a vfunc anymore.

To fix it, simply use is_multiday() function and get rid of the
vfunc.

src/gcal-subscriber-view.h

@@ -53,7 +53,6 @@ struct _GcalSubscriberViewClass
   void       (*event_activated)   (GcalSubscriberView *subscriber_view, GcalEventWidget *event_widget);
 
   /*< public >*/
-  gboolean  (*is_child_multicell) (GcalSubscriberView *subscriber_view, GcalEventWidget *child);
   guint     (*get_child_cell)     (GcalSubscriberView *subscriber_view, GcalEventWidget *child);
   void      (*clear_state)        (GcalSubscriberView *subscriber_view);