I am using free Cloudflare CDN through cPanel only. Beside that, there is one item under "Drop-ins" in my wp admin's Plugins. It displays as below:
Drop-ins are advanced plugins in the wp-content directory that replace WordPress functionality when present.
Plugin - Description
advanced-cache.php - Advanced caching plugin.
I don't know what it means or how it got there and there is no option for deactivating.