DOC: remove references to the outdated architecture.txt

As mentionned in bug #405 we continue to reference architecture.txt from
places in the doc despite this file not being packaged for many years.
Better drop the reference if it's confusing.

(cherry picked from commit 9ef75ecea129e3f77aaeb38cf5c163d0eb73742e)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 55a92393572eeff4cee4d4de738078df83e42f98)
Signed-off-by: Willy Tarreau <w@1wt.eu>
diff --git a/doc/intro.txt b/doc/intro.txt
index a60844c..ff3b3db 100644
--- a/doc/intro.txt
+++ b/doc/intro.txt
@@ -85,9 +85,6 @@
   - configuration.txt : the reference manual details all configuration keywords
     and their options. It is used when a configuration change is needed.
 
-  - architecture.txt : the architecture manual explains how to best architect a
-    load-balanced infrastructure and how to interact with third party products.
-
   - coding-style.txt : this is for developers who want to propose some code to
     the project. It explains the style to adopt for the code. It is not very
     strict and not all the code base completely respects it, but contributions