Removed share/python from PATH to fix brew doctor

Was getting the following warning from `brew doctor`:

    Warning: /opt/boxen/homebrew/share/python is not needed in PATH.
    Formerly homebrew put Python scripts you installed via `pip` or `pip3`
    (or `easy_install`) into that directory above but now it can be removed
    from your PATH variable.
    Python scripts will now install into /opt/boxen/homebrew/bin.
    You can delete anything, except 'Extras', from the /opt/boxen/homebrew/share/python
    (and /opt/boxen/homebrew/share/python3) dir and install affected
    Python packages anew with `pip install --upgrade`.

This PR clears out the contents of `python.sh` but does not remove it in case other env variables need to be set in the future.