build: Combine duplicate urlmap.js files

Seems no point in keeping them separate. It doesn’t seem to matter if
they contain entries which are unused for a particular docs build.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

docs/reference/urlmap.js

@@ -0,0 +1,9 @@
+// SPDX-License-Identifier: LGPL-2.1-or-later
+// SPDX-FileCopyrightText: 2023 Matthias Clasen
+var baseURLs = [
+    [ 'GLib', 'https://docs.gtk.org/glib/' ],
+    [ 'GModule', 'https://docs.gtk.org/gmodule/' ],
+    [ 'GObject', 'https://docs.gtk.org/gobject/' ],
+    [ 'Gio', 'https://docs.gtk.org/gio/' ],
+    [ 'Gtk', 'https://docs.gtk.org/gtk4/' ],
+];