remove UndoT

As discussed with @gseitz. Re-appears in https://github.com/larsrh/scalaz-contrib/commit/027ca08dd6d103da1dc709e499246ca843369d6e.

> < gseitz> larsrh: seems like scalaz-contrib is a better place for the undo package
> < gseitz> or I guess just drop it altogether
> < gseitz> it was just something I wanted to see if I can implement it, and I did it inside of scalaz, just because there was no snapshot published anywhere at that time
> < gseitz> larsrh: it's a port of this: http://hackage.haskell.org/packages/archive/Hedi/0.1.1/doc/html/Undo.html
> < larsrh> gseitz: oh, right. We could move it to contrib, sure.
> < larsrh> gseitz: but I probably won't move anything on my own
> < larsrh> don't want to remove something what people are using