py/modstruct: Remove unreachable code, and add comment about CPy diff.

The deleted code is unreachable because calcsize_items guarantees that
num_items corresponds to how many items there are in fmt to unpack.

Author: dpgeorge

py/modstruct.c

@@ -156,9 +156,6 @@ STATIC mp_obj_t struct_unpack_from(size_t n_args, const mp_obj_t *args) {
     }
 
     for (uint i = 0; i < num_items;) {
-        if (*fmt == '\0') {
-            break;
-        }
         mp_uint_t sz = 1;
         if (unichar_isdigit(*fmt)) {
             sz = get_fmt_num(&fmt);
@@ -191,6 +188,7 @@ STATIC void struct_pack_into_internal(mp_obj_t fmt_in, byte *p, byte* end_p, siz
     for (i = 0; i < n_args;) {
         mp_uint_t sz = 1;
         if (*fmt == '\0') {
+            // more arguments given than used by format string; CPython raises struct.error here
             break;
         }
         if (unichar_isdigit(*fmt)) {