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

This reverts commit 739123c.

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 <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Jonathan Amsterdam <[email protected]>

Author: andybons