I know what you mean about streamlining and just using 1 platform. That said, no - if you bought it on impulse or at retail, its only available through impulse. If you bought it on steam, its only available on steam. Sorry!
Remember, though - you only need to open impulse if you are updating Demigod. You can always just launch it from the desktop if you made a shortcut or through my programs. No need to open impulse at all if you are up-to-date.