lib: add return value for DC channel.unsubscribe

PR-URL: https://github.com/nodejs/node/pull/40433


Reviewed-By: Vladimir de Turckheim <vlad2t@hotmail.com>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Bryan English <bryan@bryanenglish.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>