Unlinking a platform account from your profile requires you to manually authenticate it to remove it. Bungie is not able to help with removing platform accounts. You'll want to reach out to Steam support to see if you can regain access to that account. If that's not successful and you don't have Cross Save enabled, you can link a different platform account (such as PlayStation or Stadia), sign out and back in to bungie.net via that new account, unlink your good account(s), then create a new Bungie profile with the accounts you want to use with Cross Save. This will result in losing any promotional emblems you've redeemed on this profile. If you already have Cross Save enabled with this deleted account, unfortunately there's nothing you can do, as Cross Save requires you to authenticate all linked accounts to disable it and change any accounts.