NetPlay: Refactor some functions into a common header

Source/Core/Core/NetPlayServer.h

@@ -120,8 +120,6 @@ private:
   bool SyncSaveData();
   bool SyncCodes();
   void CheckSyncAndStartGame();
-  bool CompressFileIntoPacket(const std::string& file_path, sf::Packet& packet);
-  bool CompressBufferIntoPacket(const std::vector<u8>& in_buffer, sf::Packet& packet);
 
   u64 GetInitialNetPlayRTC() const;