Replace atf-devel with atf-discuss

The new atf-discuss list lives outside of NetBSD.org to better illustrate
that ATF is not tied to a specific operating system.  The new name is to
follow the convention used in the naming scheme of the Kyua lists.
Remove stale release scripts and documentation

We now just rely on "make distcheck" to generate a release and on Travis CI
to validate our code.
Remove stale references to atf-run and atf-config

Fixes #19.