Check your theme.liquid file in the theme editor. That's usually where app code is added.
Shopify revokes access the second you delete an app, so they can't auto remove it. If you can't find the code, email the app developer, they can tell you how to remove it. Also a nice move is to recommend them to auto-send an email on app delete, with uninstall instructions so others aren't stuck with old app code loading in the future. All app devs should be doing that.