Remove cproc from the list of active maintainers
Recent years Christian Prochaska has only been making the official msys1 Windows release builds. There's no more msys1 builds coming (at least there is no such builds coming from S3_0+), and he does not want to take new responsibilities instead. We should remove his name from the list of active maintainers.
Thank you for everything you've done for Freeciv.