Some lamp posts were gas fed, most were electric. In either case you need to disconnect the gas line or electric line at the feed point (usually at the house). Removal is usually a code requirement - plus it's the smart thing to do. Abandoned gas or electric lines that are left connected can be very dangerous. If you cannot track the burried line, hire a professional who can.
Abandoned gas lines can easily leak. Even a slow leak can kill trees and grass over a large area and they are a fire hazard. Abandoned electric lines can cause circuit breakers to randomly trip and pose an electric shock hazard. Once disconnected, have fun, it's tractor pull time.