appmenu: Remove "New Event" entry

The "New Event" menu item is confusing and misleading, since
it is contextless item that has a context-aware action. The
result varies depending on which view you are, but this is in
no way communicated.

So just remove that, and all the GcalWindow API that is not
used anymore altogether.

src/gcal-window.h

@@ -35,8 +35,6 @@ G_DECLARE_FINAL_TYPE (GcalWindow, gcal_window, GCAL, WINDOW, GtkApplicationWindo
 GtkWidget*           gcal_window_new_with_date                  (GcalApplication     *app,
                                                                  icaltimetype        *date);
 
-void                 gcal_window_new_event                      (GcalWindow          *self);
-
 void                 gcal_window_set_search_mode                (GcalWindow          *self,
                                                                  gboolean             enabled);