You cannot toggle it but you can use mods/ folder, where you can add your remaked shadow.bmp(make it completely empty or transparent). So everybody who join your server will be used to this sprite, instead of their.
In this case you can add custom shadow image if you need to enable it.
you can't to do that, shadow.bmp don't change in game shadow sprite, because game use compiled sprite from exe file. Some time ago when I work with "camera" script I try to remove in some different ways. So at this moment you just can try to change shadow opacity settings to 0.
So you can get transparent shadow sprite like that:
P.s. player always in center of screen, I just change it's sprites and effects to transparent.