Removed some unused variables

Hello, I removed some unused variables and fixed two undefined variables. (Shameless plug: I detected them while testing [luacheck](https://github.com/mpeterv/luacheck)). 

Also, it looks like [moonscipt.parse.extract_line](https://github.com/leafo/moonscript/blob/master/moonscript/parse.lua#L127-132), while exported, is not used anywhere and it is not documented, so I guess it can be removed, too.