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.

src/hangulin.h

@@ -1,12 +0,0 @@
-#ifndef NEOVIM_HANGULIN_H
-#define NEOVIM_HANGULIN_H
-/* hangulin.c */
-int hangul_input_state_get(void);
-void hangul_input_state_set(int state);
-int im_get_status(void);
-void hangul_input_state_toggle(void);
-void hangul_keyboard_set(void);
-int hangul_input_process(char_u *s, int len);
-void hangul_input_clear(void);
-/* vim: set ft=c : */
-#endif /* NEOVIM_HANGULIN_H */