cleanup: drop the LIBDIR related code This code is no longer used as the python packaging echo system evolved. This code was introduced in 10da5a1f25dd, with two feature in mind: - Mercurial may be installed into a non-standard location without having to set PYTHONPATH. - Multiple installations can use Mercurial from different locations. As a side effect it also provided performance improvement at a time where the `sys.path` could be greatly inflated from setuptools `.pth` files. And it also protected from incompatible directory within the `$PTYHONPATH` variable. Both of these benefit has faded overtime as `.pth` are less common and `$PYTHONPATH` is less used (as both where creating issue to more than just Mercurial). The initial motivation (easily install Mercurial anywhere), can now be handled by a new generation of tool like pipx or uv, so it is less of a concern. Regardless of all the above, the current code is no longer used. The evolution of python packaging means that installation always go through first building a location agnostic "wheel" that cannot update LIBDIR to a proper location. Upstream packaging (debian, redhat, etc?) does not seems to adjust this variable themself. So it is safer to drop this dead code that pretend we could be doing something with it.