[cctbxbb] Removal of Boost.Thread support

Hi fellow cctbx developers, I am going to ditch support for Boost.Thread and therefore to remove the SF computation parallelised with Boost.Thread as well. Parallelisation at the C++ level should be done with OpenMP: Boost.Thread was merely an early experimentation that was kept alive for no particularly good reasons. If anybody disagrees with that move, please voice your opinion now. Best wishes, Luc J. Bourhis