How to remove "Feedback" plugin from WordPress - Step by Step Tutorial - WordPress Tutorial
If you can't find "Feedback" in your plugin list and you have JetPack installed, then this tutorial will (hopefully) show you how to remove it.
"Feedback" is not your usual plugin as you do not see it in your Plugins list. The one in my tutorial is specifically from Jetpack. It is one of the Jetpack's modules.
Troubleshooting Tip: If you have a similar problem, then try to think whether your plugin might have come as a module with another plugin.