remove the subsystem in the destructor that was added in the constructor.

This prevents nasty crashes on "environment" subsystem removal.

src/Environment/environment_mgr.cxx

@@ -60,6 +60,7 @@ FGEnvironmentMgr::~FGEnvironmentMgr ()
 {
   delete _environment;
   delete _controller;
+  remove_subsystem("controller");
   delete fgClouds;
 }