since the submodel_mgr subsystem does now also hold references to AI models,

since the submodel_mgr subsystem does now also hold references to AI models,
we need to explicitly destroy that here, too, so that it has guaranteed
access to the Nasal subsystem. Otherwise we get a segfault on exit. When
the next subsystem needs this special treatement (radar?), we should
introduce a new subsystem group (in addition to INIT and GENERAL)