Remove links to dead developer-old.gnome.org
See https://discourse.gnome.org/t/retirement-of-developer-old-gnome-org/20991
https://gitlab.gnome.org/GNOME/glib/-/blob/main/docs/reference/gio/gdbus-codegen.rst has two dead links to non-existing gtk-doc <https://developer-old.gnome.org/gtk-doc-manual/stable/>