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
I think this should not force regeneration - if the chunk is already in the generator queue, this could force the chunk to be generated twice. I'd go for a `false` param here, unless you see a specific reason to force-generate.