A mildly abrasive metal polish should do the trick, for example brasso (but I would try toothpaste first if you have no metal polish), but that is just a ground (and not strictly needed for USB function since there is another pin 4 ground on the contacts), so if it won't work you probably have a different problem.
I have used a Fibreglass bristle cleaning pen in the past to clean contacts. It works really well. Was surprised to see that they are still available. Amazon have some as do RS Components and Hobbycraft.
Keep in mind that the contacts themselves are normally gold plated and that plating will be damaged by most abrasive cleaning options, and being gold, shouldn't have oxidized itself so should be more of a fluid suspension cleaning process.