Remove obsolete versions of PLN related files

I removed the obsolete versions of PLN related files. Keeping them in the repository is unnecessary and adds clutter. 

They are still available for reference purposes if anyone needs to study them, https://github.com/opencog/opencog/tree/f04f0a4968ceebc238a5c599dbc7cbb1a53e5ef3 and https://github.com/cosmoharrigan/opencog/tree/pln-historical-jan-2014

All the unit tests are passing.