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/mark.c

@@ -1470,7 +1470,7 @@ void mark_mb_adjustpos(buf_T *buf, pos_T *lp)
     // double-wide character.
     if (lp->coladd == 1
         && p[lp->col] != TAB
-        && vim_isprintc((*mb_ptr2char)(p + lp->col))
+        && vim_isprintc(utf_ptr2char(p + lp->col))
         && ptr2cells(p + lp->col) > 1) {
       lp->coladd = 0;
     }