rt: rust_task_thread isn't ref counted

src/rt/rust_task_thread.cpp

@@ -22,7 +22,6 @@ rust_task_thread::rust_task_thread(rust_scheduler *sched,
                                    rust_srv *srv,
                                    int id) :
     rust_thread(SCHED_STACK_SIZE),
-    ref_count(1),
     _log(srv, this),
     log_lvl(log_debug),
     srv(srv),

src/rt/rust_task_thread.h

@@ -45,7 +45,6 @@ public:
 struct rust_task_thread : public kernel_owned<rust_task_thread>,
                         rust_thread
 {
-    RUST_REFCOUNTED(rust_task_thread)
 
     // Fields known only by the runtime:
     rust_log _log;