How to disable unread counter/badge on Dock app icon
To force all apps to stop the dock-bouncing, it's all or nothing, but you can launch the terminal and type this command:

$ defaults write com.apple.dock no-bouncing -bool TRUE
$ killall Dock


To re-enable dock-bouncing, type FALSE instead of true.

To remove badge icon, go to System Preferences > Notifications and select the app. You should then see the option to disable badge icons. Note: You may have to restart the app or restart the computer (simple logout/login should do) for the changes to take effect.

If you don't see it there, then I'd try perusing the Line Chat preferences or help menu to see if they have the option of disabling it.