Ignore. Does not work.
I don't know when the change happened, but it looks like you can delete all photos from the Photos apps, even the iTunes synced ones. It's all or nothing (per category), but should server for those who need to remove photos albums but no longer have access to the computer they synced with, or just want a quick way to clean out everything.
Go to Settings > General > Usage. When the Storage list updates, select Photos & Camera. You'll see three categories and how much space they take: Camera Roll, Photos Library, and Photo Stream. Swiping across them will give you a red Delete button.
The Photos Library should be the one to clear iTunes synced albums, and probably any and all other albums excluding the Camera Roll.
Big Caveat: I have not tested this. I don't want to delete my albums. I could sync them back later, but at 1.2 GB it would take some time. So use this at your own risk. And if you do, please let us know if it worked as expected.
I'm making a point of mentioning this because for a long time we've had to tell people that if they wanted to delete photos synced from iTunes on a computer they had to go back to iTunes and change the settings. This is still probably the better way of doing it; but for those that can't do so, or just don't want to be bothered, there looks to be an alternative.
Of course, if you delete the photos but don't change your setting in iTunes on the computer, they will probably just get synced back the next time.