minor #4878 [Book][Security] Remove out-dated anchor (xelaris)

This PR was merged into the 2.3 branch.

Discussion
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[Book][Security] Remove out-dated anchor

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3 (only)
| Fixed tickets |

I missed to remove the anchor along with the paragraph in PR #4746.

Commits
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dbbe9de [Book][Security] Remove out-dated anchor

Author: wouterj

book/security.rst

@@ -1246,8 +1246,6 @@ cookie will be ever created by Symfony):
     If you use a form login, Symfony will create a cookie even if you set
     ``stateless`` to ``true``.
 
-.. _book-security-checking-vulnerabilities:
-
 Final Words
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