You'll have to reach out to twitch support and create a ticket to recover the deleted account. Best of luck with that though. If you cannot authenticate to an account, it cannot be unlinked from your Bungie.net profile. You must be able to authenticate it to remove it, just as it was authenticated to to add it. You can attempt to recover your account through Twitch's support; if Twitch support cannot help you recover the account, then you will be unable to unlink it, and Bungie will not forcefully do it for you.