Remove wrong TODO comment in build_fee_bump

Remove wrong TODO comment in build_fee_bump

The proposed solution is bad for privacy as well.
Let's call the initial change output, which is normally shrink when you
fee bump, change#1, and the extra output aforementioned change#2 (as,
in this case, it's going to be a change output as well). If you add change#2
you might not revel change#1, but you're still revealing change#2.
You're not improving your privacy, and you're wasting money in fees.