minor #6850 Remove outdated reference (hvt)

This PR was submitted for the master branch but it was merged into the 2.8 branch instead (closes #6850).

Discussion
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Remove outdated reference

Finishes #6771

Original description:

 > In the example code of the custom error page, there was a comment reference to the `Avoiding Exceptions when Using Security Functions in Error Templates' section, that lived in the pre-2.8 documentation. That subsection is no longer there.

Commits
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21fcb72 custom error pages - Reference to security functions warning is no more in 2.8

Author: wouterj

controller/error_pages.rst

@@ -99,7 +99,6 @@ To override the 404 error template for HTML pages, create a new
     {% block body %}
         <h1>Page not found</h1>
 
-        {# example security usage, see below #}
         {% if is_granted('IS_AUTHENTICATED_FULLY') %}
             {# ... #}
         {% endif %}