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/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); }