Remove AUTHORS file

This goes back to when this library was part of golang.org/x/...; the "Google lawyers" said that an AUTHORS file was needed, but seems they changed their minds and it got removed from Go last week and decided that the git log is enough:
golang/go@774fa58

I don't think there's much value keeping it here, and it's one more thing that can be forgotten (it's already out of date). The authorship is preserved in the git log, and if that's good enough for Google's lawyers then it's good enough for me.