Remove unused import

Signed-off-by: Tran Ngoc Nhan <ngocnhan.tran1996@gmail.com>

core/src/main/java/org/springframework/security/access/expression/SecurityExpressionRoot.java

@@ -30,7 +30,6 @@ import org.springframework.security.authorization.AuthorizationResult;
 import org.springframework.security.authorization.DefaultAuthorizationManagerFactory;
 import org.springframework.security.core.Authentication;
 import org.springframework.util.Assert;
-import org.springframework.util.StringUtils;
 import org.springframework.util.function.SingletonSupplier;
 
 /**