minor #5278 remove unnecessary code (karion)

This PR was merged into the 2.6 branch.

Discussion
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remove unnecessary code

Commits
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8fd9a46 remove unnecessary code

Author: wouterj

reference/constraints/Callback.rst

@@ -209,8 +209,6 @@ You can then use the following configuration to invoke this validator:
             constraints:
                 - Callback: [Vendor\Package\Validator, validate]
 
-        .. code-block:: xml
-
     .. code-block:: xml
 
         <!-- src/Acme/BlogBundle/Resources/config/validation.xml -->