Remove all occurences of the mb_ptr2char macro

First step towards implemening issue #7401.

The same can be done for all deprecated mb_ functions in follow-up
patches.

src/nvim/fold.c

@@ -1767,7 +1767,7 @@ char_u *get_foldtext(win_T *wp, linenr_T lnum, linenr_T lnume,
         int len;
 
         if (has_mbyte && (len = (*mb_ptr2len)(p)) > 1) {
-          if (!vim_isprintc((*mb_ptr2char)(p)))
+          if (!vim_isprintc(utf_ptr2char(p)))
             break;
           p += len - 1;
         } else if (*p == TAB)