While you still have to delete them one at a time, if you pinch in on the page so you get to the thumbnail view it's a little bit quicker to close them one at a time; either by tapping on the x or swiping them to the left.
At least it feels that way to me.
Though, it's not something I have to do a lot. I tend to delete tabs as I go along, whenever they're no longer needed.