Doc: Remove outdated reading list

These books were published between 1995 and 1997, and are very dated.
Newcomers should be directed to learn about C++11's std::thread instead
of the platform-specific APIs.

We don't want to continue maintaining this list, as it's very easy for
users to find their own introductory reading material.

Change-Id: I780f58630e6bbebf01e89a9382832051cc3a1444
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>