You should remove the code from your theme files.
After uninstalling app, the app loses access to your store so cannot do cleanup of the theme code.
Here's how to fix:
1) Duplicate your theme and work on the backup theme.
2) Locate the app code and delete it.
3) Preview the theme and if it looks good, publish.
To find the code, either contact the app developer or go through your theme files for the template. For example, if it's on your product page, check your templates / sections / snippets that include the word "product". I'd guess this will be in product.liquid or a snippet like product-form.liquid.
If you've uninstalled a lot of apps, you can order a Tune Up from my team here (we optimize Shopify sites): speedboostr.com/services. That will handle this plus more to clean your theme up.
If you just want this one problem solved and don't ant to spend time on it, contact us we can handle it for you.