Revert "errors: add example showing a custom error with Unwrap"

This reverts commit 739123c3.

Reason for revert: broke Windows and Plan 9 builders

Fixes #33828

Change-Id: I1d85c81549b1b34924fdd0ade8bf9406e5cf6555
Reviewed-on: https://go-review.googlesource.com/c/go/+/191742
Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

src/errors/example_unwrap_test.go

-// Copyright 2019 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-package errors_test
-
-import (
-	"errors"
-	"fmt"
-	"os"
-	"time"
-)
-
-// MyError2 is an error implementation that includes a time, a message, and an
-// underlying error.
-type MyError2 struct {
-	When time.Time
-	What string
-	err  error
-}
-
-func (e MyError2) Error() string {
-	return fmt.Sprintf("%v at %v: %v", e.What, e.When, e.err)
-}
-
-// Unwrap returns e's underlying error, or nil if there is none.
-func (e MyError2) Unwrap() error {
-	return e.err
-}
-
-func readConfig() error {
-	if _, err := os.Open("non-existing"); err != nil {
-		return MyError2{
-			time.Date(1989, 3, 15, 22, 30, 0, 0, time.UTC),
-			"reading config file",
-			err,
-		}
-	}
-	return nil
-}
-
-func Example_unwrap() {
-	if err := readConfig(); err != nil {
-		// Display the error.
-		fmt.Println(err)
-		// If we can retrieve the path, try to recover
-		// by taking another action.
-		var pe *os.PathError
-		if errors.As(err, &pe) {
-			restoreFile(pe.Path)
-		}
-	}
-	// Output: reading config file at 1989-03-15 22:30:00 +0000 UTC: open non-existing: no such file or directory
-}
-
-func restoreFile(path string) {}