Since you're not reusing the glass, clamp it in a vise between two blocks of wood. and then, using another smaller block of wood and a hammer, lightly tap at the edges of the frame until you see movement. Then work your way around the frame being careful not to stress the corners until you have the frame clear of the glass. I've removed countless frames from door and vent glass this way. Of course, if you plan to reuse the glass it may damage it.