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 #53961.

Change-Id: I6f3becd863672356de571303ff31434f517d959e
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/418902
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: David Chase <drchase@google.com>