There isn't really anything to remove. Windows 7 will find whichever other computers are connected to the network. If the Laptop has been sold, it is no longer there to be found as part of the network.
Nothing is actually "installed" when another computer is added to the network.
Having said that, one of the vagaries of Windows networks is that an icon can continue to appear for a computer that is either not running or not connected, but this should fairly quickly disappear.