Its still there, its just in a really stupid location compared to how easy it was to change it before.
Go to Control Panel > Hardware & Sound > Adjust Screen Brightness (Under Power Options). There is a slider at the bottom.
If its somewhere else too hopefully someone can point out. If not, that's a dumb move on their part and hopefully they will add back to the notification center. A lot of the shortcuts on my surface pro keyboard cover don't work anymore either - like invoking the old charms bar.
Be sure to leave feedback for MS so they know your opinion.