remove yams.xy if no one is using it
see open-source/yams#7 and https://forge.extranet.logilab.fr/cubicweb/cubicweb/-/blob/branch/4.x/cubicweb/xy.py
We also need to remove cubicweb/xy.py file in the CubicWeb project.
It seems to be used in the cubes:
- sioc
- skos
- conference
- prov
- blog
- web