For security reasons, Bungie does not link or unlink accounts on behalf of players. For the account to be linked on your profile means it existed at one point and you authenticated it at that time to add it. You will need to work with Steam to recover those credentials. If you are unable to recover the credentials for the account in question then you will be stuck with this Steam account on your current profile. In that situation your only choice is to move your good accounts to a new Bungie profile so you can enable cross save there.
Bungie cannot unlink accounts from users' profiles. You will need to work with Steam to get the login information for that account. If you're unable to proceed, you have another option, which is to create a new bungie.net profile, link the correct Steam to that one, and move your other account(s) there. Doing this will result in losing any redeemed emblems from this Bungie profile.