Remove cleanup functionality from loader

Since junction check-outs are now persistent, there is no need to clean
up temporary directories once finished.
Now, junction checkouts are stored within the top-level project dir and
will be cleaned-up when that directory is removed.

This involves changes in:
* loader.py: Remove cleanup logic and passing-around of tmpdir.
* _project.py: Remove passing around tmpdir and calling loader's
               cleanup.