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