How to remove App Service Certificate resource

Go to azure resource portal (https://resources.azure.com ) and navigate to subscriptions --> specific subscription --> providers --> Microsoft.Web --> certificates and see if it is here and i think you can delete it from here directly.

Delete a certificate in Azure Resource Explorer

1. Access to: resources.azure.com
2. Expand "Subscriptions"
3. Expand "Resource groups"
4. Look and expand the resource group where the certificate is
5. Expand "Microsoft.web"
6. Click on "Certificates"
7. Choose the certificate you want to delete
8. Make sure you have selected on the top read/write and then hit on "Delete"