Remove Debian packaging files from master branch

Following the argument in the [Debian Mentors FAQ][0], the Debian packaging
files should not be present in upstream releases. They can however live on a
dedicated `debian` branch, which would also ease building the package with
[`git-buildpackage`][1]. You also may or may not check out the `debian` branch in
my repo, in which I have packaged the current v0.3.0.

[0]: https://wiki.debian.org/DebianMentorsFaq#What.27s_wrong_with_upstream_shipping_a_debian.2F_directory.3F
[1]: https://honk.sigxcpu.org/piki/projects/git-buildpackage/