Remove unused GetType() from OverrideStream, CVectorWriter, SpanReader

GetType() is never called, so it is completely unused and can be
removed.

src/netmessagemaker.h

@@ -19,7 +19,7 @@ public:
     {
         CSerializedNetMsg msg;
         msg.m_type = std::move(msg_type);
-        CVectorWriter{ SER_NETWORK, nFlags | nVersion, msg.data, 0, std::forward<Args>(args)... };
+        CVectorWriter{nFlags | nVersion, msg.data, 0, std::forward<Args>(args)...};
         return msg;
     }