I don't know how to remove the installed sticker in Viber list. Mine is quite long now.
It's not yet supported.
From viberfaq (answer posted a month ago):
At the moment it is unfortunately not possible. You can uninstall Viber, and when you reinstall, Stickers will be gone from your Sticker folder (but this will unfortunately result in message history deletion).
In the future we plan to add a feature that lets you remove Stickers from your folder.