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

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;
 

src/wallet/interfaces.cpp

@@ -578,7 +578,6 @@ public:
         m_context.scheduler = &scheduler;
         return StartWallets(m_context);
     }
-    void flush() override {}
     void stop() override { return StopWallets(m_context); }
     void setMockTime(int64_t time) override { return SetMockTime(time); }
     void schedulerMockForward(std::chrono::seconds delta) override { Assert(m_context.scheduler)->MockForward(delta); }