how do I remove a passkey

And that credential autofill item you want removed is the passkey one, correct? First I recommend signing into the service, and removing the passkey from your account options there, to not end up potentially locked out, if the service still thinks you're in possession of that credential. Afterwards you need to go to your system credential provider (might be a password vault service, or the system credential settings), and remove the passkey stored in there, e.g.: support.microsoft.com/en-us/accounts-billing/security/manage-your-saved-passkeys