Does anyone know how to remove it from being used to reset the control option to the default?
I need to remove that setting because I'm trying to set the MMB for FORWARD movement, in the ON FOOT settings, because of my disability (I don't have a left arm).

Not directly but I did find a work-around. In my case it was to put the Plasma Grenade onto the MMB (where it belongs, damn it!!). Nothing in-game or fiddling with the XML file worked but I belatedly remembered that my "Razor DEATHADDER CHROMA" mouse (silly name) had a remapping app itself.

That mouse has two side buttons (under the thumb position) and I used its app to make one of them report as the MMB and the real MMB to behave like the ` (left tick) key, which I mapped to the grenade. Now it works like it's supposed to (damn it!!).

So ... You should be able to do the same with that mouse or a similar one which supports remapping. Put the MMB function on one of the side keys and map the mouse's MMB to the forward motion key. Or, maybe, put Forward onto the front side button, MMB to the back one, and map your grenade's key to the actual MMB (like it should be, damn it!!).

I think that there are other mice out there which have more elaborate side key options.

I also would like to know the answer to that question. But, as dashiichi, I have a 5-button mouse, and for me, the thumb-buttons are for moving forward/backward (I find this to be very helpful with vehicles).

I'm currently using the Mouse with the MMB mapped to the "W" using the Logitech Gaming Software (my mouse is a Logitech G300S with 9 buttons). The problem with this is that if I try doing something with the mouse outside the game, the MMb won't work because it's mapped to the "W".

Note. The Logitech Gaming Software continually fails to recognise the game I'm playing, even though I have it set to auto detect them, so I have to manually select the profile for each game.