How should I seal a drain waste line that I'm no longer using?
There are different ways this could be sealed easily and correctly. One way would be a rubber overcover with stainless steel band clamp.
I would not remove this sewer line from the ground unless it is corroded or likely to corrode to the point of leaking. At some later date someone might wish to install an outside sink, e.g., for a fish cleaning station, dog washing,  or other use which is incompatible with an inside sink. Cut it off out of the way, and cap it properly.