To go back to the default/generic icon, begin the process as you would for an icon change (i.e., Get Info...). Click on the icon present in the Get Info box, hit the Delete key and the original will take its place. You may have to then close the Get Info window for this to happen.

On a Mac since 1984.
Currently: 24" M1 iMac, M2 Pro Mac mini with 27" BenQ monitor, M2 Macbook Air, MacOS 14.x; iPhones, iPods (yes, still) and iPads.