Can't unbind mouse keys

I wan't to bind prone and grenade throw to MOUSE 5 and MOUSE 4 respectively, but these are already taken up by Gadget One and Gadget Two, and the menu is not allowing me to unbind them. I am therefore equipping my gadget every time I prone, and I can't change it. Is this a bug or is there another way to fix it?

BTW: I play on ps5 but with mouse and keyboard.