Remove TODO

The bugzilla can be used instead. The TODO was outdated.

In order:
- Devhelp has now a GtkHeaderbar (so the GNOME 3 style is done).
- The search within document is on top.
- For online download of books, see:
https://bugzilla.gnome.org/show_bug.cgi?id=761284
- The library is used by gnome-builder too, and Anjuta is no longer well
maintained.