I totally see your point and I sympathize. Sometimes you just want a simple bike. Me too.
But please consider the derailleur is less than $30 and replacing it is probably the cheapest fastest least labor intensive way to go. The gear cluster is intertwined with the electric power cable in the rear wheel so replacing it is not the easiest thing to do.
Also, a real one speed bike still needs a chain tensioner. How would you deal with the shifter, removing it and the shift wire is some work. And if you are keeping it, you might as well get the bike into shift ready configuration just for good looks.
You don?t have to use the derailleur. You can fine tune it to work optimally with the one gear you are using. You don?t even have to connect the shift cable, just use the limiting screws. But as I said earlier, just keeping it working would make your bike look better.