message: Remove obsolete type alias
diff: @@ -29,9 +29,6 @@ class actor_proxy_impl : public actor_proxy {
  public:
  using super = actor_proxy;
  - using serialize_fun = std::vector<char> (*)(actor_system&,
  - const type_erased_tuple&);
  -
  actor_proxy_impl(actor_config& cfg, endpoint_manager_ptr dst);
  ~actor_proxy_impl() override;
  