minor #4391 Removed unused use UsernameNotFoundException (boekkooi)

This PR was submitted for the master branch but it was merged into the 2.5 branch instead (closes #4391).

Discussion
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Removed unused use UsernameNotFoundException

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.4, 2.5, master
| Fixed tickets | -

Commits
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4e6256b Removed unused use UsernameNotFoundException

cookbook/security/api_key_authentication.rst

@@ -31,7 +31,6 @@ value and then a User object is created::
     use Symfony\Component\Security\Core\Authentication\Token\PreAuthenticatedToken;
     use Symfony\Component\HttpFoundation\Request;
     use Symfony\Component\Security\Core\User\UserProviderInterface;
-    use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;
     use Symfony\Component\Security\Core\Exception\BadCredentialsException;
 
     class ApiKeyAuthenticator implements SimplePreAuthenticatorInterface