fix undoTransaction

simple bug in undoTransaction ("walletAddress +=" where it should be "walletAddress.balance +=")

Also even though undoTransaction makes the unconfirmed amount go away and the wallet page go pack to normal the non-broadcasting transaction still stays around forever in the popup so I had it remove the task

I'm just getting started with the darkwallet code so feel free to change it if made the fix in the wrong place/coding style/whatever