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/ex_cmds.c

@@ -5641,7 +5641,7 @@ void ex_sign(exarg_T *eap)
             // Count cells and check for non-printable chars
             cells = 0;
             for (s = arg; s < p; s += (*mb_ptr2len)(s)) {
-              if (!vim_isprintc((*mb_ptr2char)(s))) {
+              if (!vim_isprintc(utf_ptr2char(s))) {
                 break;
               }
               cells += (*mb_ptr2cells)(s);