minor #12863 Update user_checkers.rst (atailouloute)

This PR was merged into the 4.3 branch.

Discussion
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Update user_checkers.rst

Remove unused import

Commits
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aa34b59 Update user_checkers.rst

Author: javiereguiluz

security/user_checkers.rst

@@ -23,7 +23,6 @@ are not met, an exception should be thrown which extends the
     use App\Exception\AccountDeletedException;
     use App\Security\User as AppUser;
     use Symfony\Component\Security\Core\Exception\AccountExpiredException;
-    use Symfony\Component\Security\Core\Exception\CustomUserMessageAuthenticationException;
     use Symfony\Component\Security\Core\User\UserCheckerInterface;
     use Symfony\Component\Security\Core\User\UserInterface;