my new below-entry-level MTB came with a basic coil-spring front suspension, with 2 super firm/rigid springs. 2 new soft springs would cost like $50, so I was wondering, can you simply remove a spring from one side, thus effectively halving the spring rate?
No, i would not advise doing so. If it is a dual spring fork like your image illustrates one spring-less leg would put the entire thing under constant stress. This seems like a horrible idea all around. It may loosen up some after time, and you may be able to adjust some settings if that is the same fork in the image.
Does is have any knobs on the top of the legs like those in the image?
I would advise either buying softer springs if available, a new fork, or just learning to deal with it. Inexpensive fork are not really made to be adjusted or tampered with generally and i would not trust them to perform after doing so.
I'm inclined to think that removing one fork spring will not work very well, and I definitely think there is significant potential for catastrophic and dangerous failure of the fork.
The side with the spring will experience a resistance force but the side without the spring will not. The axle will therefore only be supported on one side, which will place stress on the slider axle dropouts and axle itself. The alloy slider dropouts could crack and break off.
if that happens You'll lose the front wheel and go over the bars. Having seen (and helped) a couple of people who face-planted into tarmac I can tell you you don't want to do it, no matter how good your dental insurance is.