Remove dead code in DefaultErrorAttributes

See gh-27538

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/servlet/error/DefaultErrorAttributes.java

@@ -36,7 +36,6 @@ import org.springframework.util.ObjectUtils;
 import org.springframework.util.StringUtils;
 import org.springframework.validation.BindingResult;
 import org.springframework.validation.ObjectError;
-import org.springframework.web.bind.MethodArgumentNotValidException;
 import org.springframework.web.context.request.RequestAttributes;
 import org.springframework.web.context.request.WebRequest;
 import org.springframework.web.servlet.HandlerExceptionResolver;
@@ -219,9 +218,6 @@ public class DefaultErrorAttributes implements ErrorAttributes, HandlerException
 		if (error instanceof BindingResult) {
 			return (BindingResult) error;
 		}
-		if (error instanceof MethodArgumentNotValidException) {
-			return ((MethodArgumentNotValidException) error).getBindingResult();
-		}
 		return null;
 	}