You can see the end of the pin where the C clip was, this pushes away from you (into the case). So using the flat blade screwdriver, go to the other end of the pin (which is shape like a T) and lever the pin towards the inside of the case. There are two (nylon?) washers / bushes, make sure you don't loose them. One of them may get trapped inside the C-clip groove, so them to make sure the pin is centred as it passes through.
Many thanks to the OP, FINALLY stripped my G5. Now contemplating remove ALL those screws to get access to the back of the motherboard posts.