message: Fix unused-variable warning
diff: @@ -33,8 +33,6 @@ using atomic_count = std::atomic<size_t>;
  size_t assumed_thread_count;
  size_t assumed_init_calls;
  -std::mutex mx;
  -
  struct dummy_thread_hook : thread_hook {
  void init(actor_system&) override {
  // nop
  