[RFC] Remove hangul input support

Vim [documentation](http://vimdoc.sourceforge.net/htmldoc/hangulin.html), says
that hangul support is scheduled to be removed. I think it's safe to say we
don't want to support a feature even vim is considering removing.

Everything still compiles even after removing the header, so it's not being
used.

Before doing the initial import to neovim's repository, I had to tweak this
module to make it compile for terminal. It was a mistake that is now being
corrected.