ssl_stream does not use flat_stream

We no longer need to use flat_stream to improve write performance, as
Asio now linearizes scatter/gather I/O in ssl::stream:
https://github.com/chriskohlhoff/asio/commit/17637a48ccbfa2f63941d8393a7c8316a8df4a79

example/websocket/server/sync/websocket_server_sync.cpp

@@ -108,9 +108,9 @@ int main(int argc, char* argv[])
             acceptor.accept(socket);
 
             // Launch the session, transferring ownership of the socket
-            std::thread{
-                do_session,
-                std::move(socket)}.detach();
+            std::thread(
+                &do_session,
+                std::move(socket)).detach();
         }
     }
     catch (const std::exception& e)