In the attached picture you can see the radiator in my second floor bathroom. The hot water in pipe is dark because I've removed the paint. But you can see a white pipe that goes nowhere coming up behind the radiator. Is that pipe important? Is it something I'd open to bleed air out of when refilling the system? Is it an artifact left over from some previous configuration? Maybe running to an old and now missing expansion tank? Can I get rid of it? I'd like to replace the entire configuration with something that allows for a sink that fits the space a bit better.
Possibly relevant details: It's a closed system hot water radiator system with the expansion tank in the basement. It's gas based, but converted from oil. This is on the top floor so that pipe is the highest point in the radiator system.