I've removed one before from a junkyard car. If I remember correctly we removed the clip. Member Jncomutt and I used a piece of spiral wire from home depot and two bolts to saw all the way around the glass, from the inside and out. Not one of the better memories.
A somewhat ironical occurrence, when towing this car to the scrap yard:

The air pressure going down the highway was enough to press the rear window out. I looked out the back of the truck and didn't see the window, stopped, and found it laying down nicely in the engine bay. It was attached to the car by the rear defroster power and ground wire. That glass is wrapped up nicely in storage. I don't really care to saw another one out if a don't have too. Putting the new glass back in is easy, just be sure to space the bottom of the glass correctly. I used the stock rear glass wedges to hold it in place while the the glue dried.