Remove parent parameter from write_serialization

This is no longer needed as each PDU knows its parent PDU already

include/tins/snap.h

@@ -177,7 +177,7 @@ private:
         uint16_t eth_type;
     } TINS_END_PACK;
     
-    void write_serialization(uint8_t* buffer, uint32_t total_sz, const PDU* parent);
+    void write_serialization(uint8_t* buffer, uint32_t total_sz);
     
     snap_header snap_;
 };