Don't include simgrid/config.h from simgrid/modelchecker.h so that the later is cheap...

Don't include simgrid/config.h from simgrid/modelchecker.h so that the later is cheap to load

The functions are now generated in any case, and only do something
when SIMGRID_HAVE_MC is compiled in.

Also:
 - kill MC_automaton_new_propositional_symbol() that was never implemented
 - kill MC_visited_reduction() that was not used (anymore?)
 - Convert _sg_do_model_check to simgrid::mc::cfg_do_model_check