refactor: VectorWriter without nVersion

The field is unused, so remove it.

This is also required for future commits.

src/net.cpp

@@ -818,7 +818,7 @@ bool V1Transport::SetMessageToSend(CSerializedNetMsg& msg) noexcept
 
     // serialize header
     m_header_to_send.clear();
-    CVectorWriter{INIT_PROTO_VERSION, m_header_to_send, 0, hdr};
+    VectorWriter{m_header_to_send, 0, hdr};
 
     // update state
     m_message_to_send = std::move(msg);