authhandler: Remove example_test.go

The example test implementation uses stdin, which does not work in all environments:
golang/go#45523

Removing for now to unblock release.

Change-Id: I49bafa9fe1d973b7c1d7ce00f51f110f9aa4a5a6
GitHub-Last-Rev: baf4632
GitHub-Pull-Request: #488
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/309469
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Trust: Cody Oss <codyoss@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

Author: andyrzhao