Having worked at a movie theater, I can tell you that the answer is "by a human"
If you can make a machine that cleans a popcorn popper well enough to pass a health inspection, you'll be a very rich person. Especially a small vending-machine-sized popper-and-dispenser all-in-one jobby.
Pretty sure a machine that can be built to make popcorn can also.be engineered to self clean. Human maintenance one per week to swap out parts and re-top corn.
I'm sure it's possible but I suspect it's simply not cost effective.
Removing grease requires hot water, harsh solvents, and scrubbing. If you've ever cleaned an oven or bbq or whatever I'm sure you'd agree that it would be very difficult to automate.