Remove interals2 book

The "Hacker's guide" is grossly outdated (it is about ZendEngine 2;
some stuff even about ZE1, although ZE4 is current), and nobody is
working on it for years.  Therefore we remove this part of the manual;
users are better off reading external resources, such as
<http://www.phpinternalsbook.com/>.

Closes GH-152.

Author: cmb69