It is the fdm that is causing the transfer of fuel. The fdm is the part the pump is in and controls which tank gets the return fuel. It's called the fuel delivery module.
Why are you removing the side tank. Is it bad or what. It makes more sense to replace it and have the additional fuel available. And they aren't much money. The fdm is where the investment comes.
But if you must remove it, then cap the hoses off real well, and that's all you need to do.