Turn off the main breaker, remove the feeder wires at the bottom, along with the lugs. Don't forget to remove the "neutral wire" that is going to the neutral block on right side of main bus in picture. Cut the wires at the conduit. Reinstall the cover, then turn main breaker back on. Next I would cut the conduit and wires inside, maybe a couple inches above ground (??) and then put a plug on the end. As well, put a hole cover on bottom of panel where conduit used to go into panel. I would say it's up to you if you want to leave the wire or pull it out of the conduit . . .