* gtkutil.c: Remove no-longer-used code.

(xg_update_menubar): Remove unused local and unnecessary call to
gtk_menu_item_get_submenu.
(XG_TOOL_BAR_PROXY_BUTTON, xg_tool_bar_proxy_callback)
(xg_get_tool_bar_widgets, xg_tool_bar_proxy_help_callback)
(TOOLBAR_TOP_WIDGET): Remove; no longer used.