Delete reveal.py

"Closes DFHack/dfhack#229"

See title, [plugins/reveal.cpp](https://github.com/DFHack/dfhack/blob/master/plugins/reveal.cpp) is a full-featured replacement for [needs_porting/reveal.py](https://github.com/DFHack/dfhack/blob/master/needs_porting/reveal.py).  The latter should thus be unceremoniously deleted.