How do I remove the back lip from paving stones?
Ok, those aren't paving stones, those are wall stones (as the Lowes page confirms) - as in retaining wall stones.  The lip is designed to hook to the stone underneath and prevent slipping when pressure is applied from the rear.

Which isn't to say you cannot use them, but you'll find removing the lip much easier if you use a good masonry/stone chisel, like such:


Put the cutting edge of the chisel at the base of the lip, holding the chisel near parallel with the brick surface, and strike the other end firmly with the hammer.  The chisel will create a very focused point of pressure which will chip the lip away. 

Finally, since its a simple fire pit, use stone adhesive to fix the blocks in place.  You shouldn't have to worry about the kind of back pressure these bricks are usually designed to withstand.