refactor: Initialize magic bytes in constructor initializer

Also remove an assert that is already enforced by the compiler checking
that the length of the std::array matches.

src/net.h

@@ -372,7 +372,7 @@ public:
 class V1Transport final : public Transport
 {
 private:
-    MessageStartChars m_magic_bytes;
+    const MessageStartChars m_magic_bytes;
     const NodeId m_node_id; // Only for logging
     mutable Mutex m_recv_mutex; //!< Lock for receive state
     mutable CHash256 hasher GUARDED_BY(m_recv_mutex);