It's unfortunate that you chose to link an account that isn't yours directly to your Bungie.net profile. As ishikusui linked, as of March of last year, unlinking a gaming platform account is no longer an option. There are no exceptions to this. If you want to play on Steam with the data linked to this Bungie.net profile, you'll need to use the Steam ID you already have linked to your Bungie.net profile. If you want to use your own Steam ID, then you'll be starting fresh, as you won't be able to link it to the save data you already have.