Fix deactivation issue with `system` version

    $ pyenv global system
    $ pyenv shell venv
    (venv) $ pyenv shell --unset
    (venv) $ pyenv version
    system (set by /home/yamashita/.pyenv/version)
    (venv) $ pyenv which python
    /home/yamashita/.pyenv/versions/venv/bin/python # <-- should be `/usr/bin/python` (system)