AFAIK it just pulls straight out. Every speedo I've ever seen is just a square cable into a square hole. Make sure you're pulling it straight out and it should come out with little effort.
Looking at your pic again, that galvanized sleeve may have a little corrosion underneath it. A smal blast of some PBlaster or Seaform DeepCreep (or whatever anti-corrosion agent you have) may be in order. VDO gauges have a threaded cap that holds the cable in place. Once you unscrew that, the cable just slips right out of the speedo.