How to remove a concrete-filled, steel bollard
Use (if you can fit it on) or replicate the function of a "lally column cutter" - I suspect replicate the function will be the better bet - use a cutoff disk in an angle grinder to score a line around the base of the steel (not a piddly little score, ideally - deepish, but it need not hit concrete) and then smack the top of the column. Enjoy the power of "stress concentration" as the steel snaps at the score-line, and the concrete will go with it.

Be sure that your new structure provides adequate protection to whatever you are building a structure over (I'm gathering the hydrant is an example, not the actual thing) if being plowed into by a vehicle remains an issue; which presumably is why there are bollards there in the first place.