Remove bundled "invaders" demo; now in edwinb/idris-demos

The canonical, maintained version of this is in
https://github.com/edwinb/idris-demos
The version in this repo is unmaintained and no longer compiles.
Removing to avoid confusion.
Merge pull request #6 from LeifW/remove_invaders

Remove bundled "invaders" demo; now in edwinb/idris-demos