How to remove the set key from shower faucet?
Looking at the drawing and photo I'd have thought you just need to get a small screwdriver or awl in the gap you've shown there, and the one on the other side and gradually work it out, ensuring it stays straight.
If you work it out a bit at a time, alternating sides, make sure to support the side you've just moved, or it may simply pivot on the middle section.