That round, black, plastic is for adjustment, up & down so the door rides level. On the upper track, look for two open slots for the wheels to pop out of the track by lifting up on the door and lining everything up.

Update: After looking more closely to your picture, I think you have to insert a lg screwdriver into that slot and turn the black insert, pop it out. Behind it lies the secret to door removal.

Artemisc is right. The top one presses into the door on a spring so you can lift the door from the bottom and pull the bottom out.

The top one retracts. Grab the door by the side, and lift it, the top wheel will get pushed in, and you can swing the bottom out.

use a jack hammer giggity

Both the top and bottom wheels are encased by the rails. This round part seems to have something to do with it, but I cannot squeeze it with pliers, turn it, or pop it out. No visible screws anywhere except in the joints between the two parts. Suggestions?