I just did this today and it was actually easy. Note that the ends of the ring are beveled- what I did was slip a small (1/8") screwdriver under the bevel on one end and pry straight toward the center of the fork slider. As soon as I had it just clear of the groove, I inserted an X-acto knife blade behind it so it couldn't snap back into the groove. Then, I moved the screwdriver to the other end of the ring, just to keep the ring from rotating in the groove as I slid the X-acto knife around the perimeter. About halfway around, the ring popped out completely.
It actually took more time to write this than to get the ring out!