Remove unused strands in server examples

example/advanced/server-flex/advanced_server_flex.cpp

@@ -1023,7 +1023,6 @@ public:
 class listener : public std::enable_shared_from_this<listener>
 {
     ssl::context& ctx_;
-    boost::asio::io_service::strand strand_;
     tcp::acceptor acceptor_;
     tcp::socket socket_;
     std::string const& doc_root_;