minor #15188 [Security] remove unused namespace (felipyamorim)

This PR was merged into the 5.2 branch.

Discussion
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[Security] remove unused namespace

when the CustomUserMessageAccountStatusException was introduced in 5.1 version, AccountDeletedException is no longer used in the sample code

Commits
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9e9796b remove namespace unused

Author: javiereguiluz

security/user_checkers.rst

@@ -24,7 +24,6 @@ displayed to the user::
     namespace App\Security;
 
     use App\Entity\User as AppUser;
-    use App\Exception\AccountDeletedException;
     use Symfony\Component\Security\Core\Exception\AccountExpiredException;
     use Symfony\Component\Security\Core\Exception\CustomUserMessageAccountStatusException;
     use Symfony\Component\Security\Core\User\UserCheckerInterface;