Hello,
Unfortunately, if you cannot authenticate the psn account, you will not be able to unlink your xbox account.
[quote]Players who can't authenticate connected platform accounts will have to contact their platform's support to try and gain access to the account.[/quote]
Bungie is not able to remove connected platform accounts from profiles under any circumstances.
More information on this issue is available [url=https://help.bungie.net/hc/en-us/articles/360049498031-Destiny-2-Cross-Save-Guide]here.[/url]