Remove client-side downloads limit check on upload

When trying to set downloads limit to more than 20 on upload, ffsend returns an error: The downloads limit must be one of: 1, 2, 3, 4, 5, 20, so we have to use --force. At the same time, most Send instances have downloads limit more than 20. This limit is enforced server-side anyway, so I don't really see a point in enforcing it with a limited set of options on the client.