The script says "She disappears."
RobbieJ is the only person that has realy given any sort of reasoning behind their answer, and his is the one that makes the most sense. It would take a director, however, to make that clear, as it's not made clear in the lyrics or stage directions.
Since Sondheim and Lapine obviously changed it a little bit in the revival, what they were going for is exactly what Robbie explained: The Witch loses the beans so that she will regain her ugly state and in turn, her powers, so that she can use them to escape. I guess that's what's supposed to happen. Whether or not that's the most effective ending of the Witch.
Eeeeeeyyyyyyyyaaaaaaaannnnnddddd aaaaaaaiiiiiiiiyyyyyyaaaaaammmmmmmm teeeeeeeelllllliiiiiinnngg yyyyooooooouuuuuuuwwwaaaahh...