Hello,
From what I can see this would be a discord issue and no one here can actually do the unlinking process for you. Sadly when you connect an account to Steam, Steam doesn't actually pay attention that it is connected the link is totally one-sided and is basically just a shortcut to your Steam profile, meaning there is also no way that you can unlink them through Steam.
The only way I believe that you could resolve this issue is to contact Discord Customer Support and if you can prove that you owned the previous Discord account then they might be able to unlink it for you. Here is a link to the Discord Customer Support page -