You can only unlink an account if you are able to authenticate it, i.e. log in with its credentials. Bungie cannot unlink anyone's accounts in their profiles due to security reasons. If you currently have no access to your Xbox account, your best option is to contact Xbox support and try to recover the account. After that you should be able to unlink it from your profile. If you are ultimately unable to authenticate the account, you will not be able to unlink it from your profile and thus not going to be able to use this profile for cross-save (as all linked accounts must be authenticated). In this case, your only option would be to create a new Bungie profile and link all your active and authenticated accounts in there. Note that abandoning your current profile will result in you losing all promotional items, such as emblems that have been redeemed using this profile. These items are not transferable.