remove dead flush()

It is confusing that the chain client flush happens between
StopHTTPServer and StopMapPort. Also, it is unused code. Seems best to
just add it back properly when it is needed again.

src/init.cpp

@@ -298,9 +298,6 @@ void Shutdown(NodeContext& node)
     StopREST();
     StopRPC();
     StopHTTPServer();
-    for (const auto& client : node.chain_clients) {
-        client->flush();
-    }
     StopMapPort();
 
     // Because these depend on each-other, we make sure that neither can be