remove the "ai_model" subsystem manually before the other subsystems

remove the "ai_model" subsystem manually before the other subsystems
are taken down by the C++ runtime environment. This will later be done
with runlevels. Why would we want to run nasal code in subsystem
destructors? We don't really. But some data structures may use nasal,
which are normally created/destroyed during runtime. And these will
also be destroyed at fgfs exit. In the past things like these didn't
happen, because someone had disabled all subsystem destructors ...