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].

[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/