minor symfony#5812 Remove duplicate and confusing info about testing error pages (carlos-granados)

This PR was submitted for the 2.6 branch but it was merged into the 2.7 branch instead (closes symfony#5812).

Discussion
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Remove duplicate and confusing info about testing error pages

We added new information about using the ExceptionController to test the error pages but the old info was not deleted. In this way, the section was duplicated and offered information which seemed to be contradictory. I have removed the old info.

Commits
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4007ba1 Remove duplicate and confusing info about testing error pages

Author: xabbuh

cookbook/controller/error_pages.rst

@@ -142,24 +142,16 @@ is undefined. The solution is to add the following check before using this funct
         {# ... #}
     {% endif %}
 
+.. _testing-error-pages:
+
 Testing Error Pages during Development
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 While you're in the development environment, Symfony shows the big *exception*
 page instead of your shiny new customized error page. So, how can you see
 what it looks like and debug it?
 
-The recommended solution is to use a third-party bundle called `WebfactoryExceptionsBundle`_.
-This bundle provides a special test controller that allows you to easily display
-custom error pages for arbitrary HTTP status codes even when ``kernel.debug`` is
-set to ``true``.
-
-.. _testing-error-pages:
-
-Testing Error Pages during Development
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-The default ``ExceptionController`` also allows you to preview your
+Fortunately, the default ``ExceptionController`` allows you to preview your
 *error* pages during development.
 
 .. versionadded:: 2.6