Drop GDK_MULTIHEAD_SAFE

We don't support multiple screens anymore, so there is no need
for marking API as multihead safe any longer.

gdk/gdkevents.h

@@ -1305,14 +1305,10 @@ void	  gdk_set_show_events		(gboolean	 show_events);
 GDK_AVAILABLE_IN_ALL
 gboolean  gdk_get_show_events		(void);
 
-#ifndef GDK_MULTIHEAD_SAFE
-
 GDK_AVAILABLE_IN_ALL
 gboolean gdk_setting_get                (const gchar    *name,
                                          GValue         *value);
 
-#endif /* GDK_MULTIHEAD_SAFE */
-
 G_END_DECLS
 
 #endif /* __GDK_EVENTS_H__ */