How do I get rid of an unnecessary firmware update?

Ah, I know exactly what you're talking about. The error you're seeing is this:

I'm not sure that "updatable" is a word (isn't it "able to be updated"?) but nonetheless, the problem with this firmware update is that the delete when done feature is missing from the "no update needed" option.

Fortunately, it's easy to find the firmware update utility and axe it. Just go to Apple --> System Preferences and choose "Accounts". Now click on "Login Items" and you'll see:

The last item is what we seek, SuperDrive Update 2.1. Click on it, then click on the "- symbol and it'll be removed from the list and will never bother you again.

If you want to free up the minimal disk space that the update needs, you can always use Spotlight to find the app and delete that too.