Remove snap download link because it is not maintained

The snap package is no longer maintained and despite multiple efforts over the last 3 months, I was not able to find a new maintainer nor were we able to organize a handover to the Veloren project. In order to prevent users from continuing to download the unmaintained package, I have set it to private and recommend to remove the download instructions. If there will be interest in maintaining it in the future (e.g. if the handover to the Veloren project will eventually take place), we can make it public again and add the download links back. For now, however, users should no longer download the unmaintained package for obvious reasons.