refactor(map): remove extra-allocating map_new/map_free functions

Note: the reason for removing them is not that there after this refactor
is no use of them, but rather that having them available is an
anti-pattern: they manange an _extra_ heap allocation which has
nothing to do with the functionality of the map itself (khash
manages the real buffers internally). In case there happens to
be a reason to allocate the map structure itself later, this
should be made explicit using xcalloc/xfree calls.

src/nvim/api/private/helpers.c

@@ -1724,7 +1724,7 @@ const char *describe_ns(NS ns_id)
 {
   String name;
   handle_T id;
-  map_foreach((&namespace_ids), name, id, {
+  map_foreach(&namespace_ids, name, id, {
     if ((NS)id == ns_id && name.size) {
       return name.data;
     }