A+C: delete AUTHORS and CONTRIBUTORS

In 2009, Google's open-source lawyers asked us to create the AUTHORS
file to define "The Go Authors", and the CONTRIBUTORS file was in
keeping with open source best practices of the time.

Re-reviewing our repos now in 2022, the open-source lawyers are
comfortable with source control history taking the place of the
AUTHORS file, and most open source projects no longer maintain
CONTRIBUTORS files.

To ease maintenance, remove AUTHORS and CONTRIBUTORS from all repos.

For golang/go#53961.

Change-Id: I182181f0c116a7218ad9aba1c7a645fe0190762a
Reviewed-on: https://go-review.googlesource.com/c/sync/+/418922
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>

Author: rsc