wallet: Remove unused db functions

SOme db functions were for BDB, these are no longer needed.

src/wallet/interfaces.cpp

@@ -578,7 +578,7 @@ public:
         m_context.scheduler = &scheduler;
         return StartWallets(m_context);
     }
-    void flush() override { return FlushWallets(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); }