refactor: remove char_u (#22829)

Closes https://github.com/neovim/neovim/issues/459

test/functional/lua/ffi_spec.lua

@@ -25,7 +25,6 @@ describe('ffi.cdef', function()
       local ffi = require('ffi')
 
       ffi.cdef[[
-        typedef unsigned char char_u;
         typedef struct window_S win_T;
         typedef struct {} stl_hlrec_t;
         typedef struct {} StlClickRecord;