You can probably find most of the old app code in your theme.liquid layout file.
Other common areas you might find extra code:
Those are the main template files, so you'll want to check there then trace the included snippets as well for a thorough check.
If you'd like a professional to handle it, we have an App Analysis service here: speedboostr.com/services that addresses this.
We see this quite often, with leftover app code. It's because after deleting an app from your Shopify store, access is removed so the app cannot clean up after itself.
Aibek is here from Growavelabs Agency: https://growavelabs.com/
I think we can help you with this problem.
We can analyze and make speed optimization for you. For free.
Hope to talk with you soon.