Remove shared tydescs

All tydescs are static now, there's no need to worry about
marshalling them between threads anymore.

src/rt/rust_task.cpp

@@ -192,7 +192,6 @@ void task_start_wrapper(spawn_args *a)
         // free the environment (which should be a unique closure).
         const type_desc *td = env->td;
         td->drop_glue(NULL, NULL, td->first_param, box_body(env));
-        upcall_free_shared_type_desc(env->td);
         upcall_shared_free(env);
     }