Update repo to reflect that `pandoc-citeproc` is now part of `pandoc`

[cthoyt]

https://github.com/jgm/pandoc-citeproc is now archived and says that its functionality is now built into pandoc, and that no filter is necessary. A lot of the tooling I've found for JOSS has references to this, and is confusing. It would be good to start removing references to it in this repo