While you may be playing the game on PC, you are using your Xbox character data to do so. Cross-save is allowing you to use your Xbox data, but that data will always be tied to your (now deleted) Xbox account. Your Steam account has no character data, and even if you could deactivate cross-save, you would be left with a blank Destiny account on Steam. Your current cross-save setup is required to remain as is, if you wish to continue utilizing the save data that you have been playing from. Cross-save does not transfer save data between platform accounts, and there is no option to permanently transfer account data from one platform to another.
Hey there! Unfortunately nothing can be done if you are not able to authenticate your account to deactivate cross save. Until you do not recover your account, cross save cannot be deactivated.