schema CHANGE remove private_destructor parameter from ly_ctx_destroy()

The mechanism was not reliable and not used often. libyang provide
function to traverse the schema and free the set data manually or the
caller can maintain some information to free the set data directly.

This commit changes API.
diff --git a/src/tree_schema.c b/src/tree_schema.c
index 0840b7e..05f8bf8 100644
--- a/src/tree_schema.c
+++ b/src/tree_schema.c
@@ -1392,7 +1392,7 @@
     goto cleanup;
 
 free_mod_cleanup:
-    lys_module_free(mod, NULL);
+    lys_module_free(mod);
     if (ret) {
         mod = NULL;
     } else {