remove y-coord from chunks

I removed the y-coord from chunks, I hope I didn't miss anything. Fixes #1344
Now you can slaughter me for removing too much. :P
Or will travis kill this commit first? :O
This requires a bit more love. The Y coord was actually used here to signalize that a chunk should be regenerated, even if it existed. You've lost this check, so you broke the `/regen` command and the ChunkWorx plugin.