They make those adapters for this purpose. The ones with the green pigtail and a spade lug on them are the easiest to use. Assuming the existing light is in a metal box, you can find a screw on there to connect the pigtail to. This also of course assumes the metal box is properly grounded.
This solution isn't perfect, but it is far better than cutting the ground prong off. If the ground prong is cut off, it is then best to never touch the fixture while it is plugged in, because if it has a hot to chassis fault in there, overcurrent protection will not trip, but does present an electrocution hazard.