libdocument: backends: remove XPS and PostScript backends

Closes #151 (closed)

There PS was discussed. We have only discussed XPS in private, but I think there's a general understanding that this is something that brings no real benefit (it was never a widespread format, and seems to have virtually no users?), apart from having some old code around. People can, in addition, convert those files to PDFs, so we'd rather ask people for that, than carry the code here. It could always be maintained as an out-of-tree module

@camelCaseNick @crab2313 thoughts?