Remove unused strands in server examples

example/http/server/async-ssl/http_server_async_ssl.cpp

@@ -379,7 +379,6 @@ 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_;
@@ -391,7 +390,6 @@ public:
         tcp::endpoint endpoint,
         std::string const& doc_root)
         : ctx_(ctx)
-        , strand_(ios)
         , acceptor_(ios)
         , socket_(ios)
         , doc_root_(doc_root)