pythonPackages.xgboost: fix build

The setup.py of `python-package` contains some path resolution magic to
find `libxgboost.so` which is needed for the python API.

Unfortunately the code is incompatible with Nix as it isn't compatible
with the store-based structure for each package and tries to express the
location of the shared object with relative paths.

The detection in `setup.py` and `xgboost/libpath.py` has been either
removed entirely or patched to link to the proper store path of the
`libxgboost` build input.

See https://hydra.nixos.org/build/77702715 for further reference.