This app has added the code and it stays there even after you uninstall it.
It shows the boxes of the discounts table that has been there.
The bottom line, stay away from this app.
I'm really sorry to hear you disliked the app. We have already sent an email to you but received no response yet.
The app does not modify Shopify themes and can be safely installed and uninstalled.
The codes you mean are temporary variants created by the app to override the product price in the cart. The variants are hidden on the Storefront and removed automatically after checkout or in 1 day if not purchased. Customers shouldn't see them when the app is active. Since you uninstalled the app simply remove the variants manually in the Shopify product.
Please contact us and we will help you fix the issue.