DFT-D3 cleanup

It is no longer useful to keep the DFT-D3 code untouched, for two good reasons:

1. it had to be modified anyway for MPI/OpenMP/OpenACC parallelization, and
2. it is superseded by github.com/dftd4/dftd4 and presumably no longer maintained.

So I have started removing unused stuff. Next steps:

• remove the dependencies upon QE modules mp, mp_images
• make the test compile and work again (now it doesn't)