Remove 2 broken links for programming reference books in Appendix I. After
a lookup on an Internet archives web site, it seems that these links have
been broken for around 3 months. We can then assume that they will not be
back up and safely remove them from the documentation.

Signed-off-by: Olivier C. Larocque <olivier.c.larocque@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>