Don't write uploaded filename and description to env variables

Environment variables are shared among all threads of a process (e.g. all nodes of a instance of sbbs), so this scheme of passing the uploaded filename and description to an external file tester (upload processor) via environment variables has always been fraught with the possibility of failure or flakiness in Synchronet v3. Since I very much doubt that any upload processor is actually using this scheme, just remove it. Upload processors can and should)use the sbbsfile.nam and sbbsfile.des text files created in the node directory instead.