Remove internals2 book

[cmb69]

The "Hacker's guide" is grossly outdated (it is about ZendEngine 2;
some stuff even about ZE1, although ZE4 is comming), 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/.

---

This is only the removal for now; we should actually add some information about external resources (some form of curated link list), but where? We should probably also remove the low-level parts of the Garbage Collection section.

We should also not forget to delete the respective user notes from the manual, and to close doc bug reports regarding internals documentation.

This PR requires the following patch to doc-base:

Index: manual.xml.in
===================================================================
--- manual.xml.in	(revision 350824)
+++ manual.xml.in	(working copy)
@@ -440,26 +440,6 @@
   </set>
  </set>
 
- <book xml:id="internals2">
-  <title>&Internals2;</title>
-  &internals2.intro;
-  &internals2.memory.index;
-  &internals2.variables.index;
-  &internals2.funcs.index;
-  &internals2.objects.index;
-  &internals2.resources.index;
-  &internals2.ini.index;
-  &internals2.streams.index;
-  &internals2.counter;
-  &internals2.buildsys.index;
-  &internals2.structure.index;
-  &internals2.pdo.index;
-  &internals2.faq.index;
-  &internals2.apiref.index;
-  &internals2.opcodes;
-  &internals2.ze1.index;
- </book>
-
  <book xml:id="faq">
   <title>&FAQ;</title>
   <titleabbrev>&FAQabbrev;</titleabbrev>

[cmb69]

PR to move the PDO Driver How-To to phpinternalsbook: phpinternalsbook/PHP-Internals-Book#97

[afilina]

Is there a TODO entry for the follow-up tasks on bugs.php.net? Of so, then I believe this is mergeable (after rebase).
I would recommend updating doc-base first and merge this one afterwards.

[cmb69]

Thanks for the reminder! It's overdue to remove this.

I have removed all user notes; roughly a third of them were regarding the ZE1 docs; another third were actually about userland stuff (e.g. there have been several user notes regarding the instanceof operator on the INSTANCEOF OPcode page).

Also, I'd like to note that I submitted a draft PR regarding internal stream documentation a while ago: phpinternalsbook/PHP-Internals-Book#98

And I have no filed https://bugs.php.net/bug.php?id=80612 to not forget about adding info about external resources regarding this topic.