refactor: add Packet::skipAll() to replace 186 setReadPos(getSize()) calls

Add skipAll() convenience method and replace 186 instances of the
verbose 'discard remaining packet data' idiom across game_handler
and world_packets.

include/network/packet.hpp

@@ -46,6 +46,7 @@ public:
         return getRemainingSize() >= guidBytes;
     }
     void setReadPos(size_t pos) { readPos = pos; }
+    void skipAll() { readPos = data.size(); }
 
 private:
     uint16_t opcode = 0;