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/interfaces/chain.h

@@ -407,9 +407,6 @@ public:
     //! Start client execution and provide a scheduler.
     virtual void start(CScheduler& scheduler) = 0;
 
-    //! Save state to disk.
-    virtual void flush() = 0;
-
     //! Shut down client.
     virtual void stop() = 0;