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/tests/fuzz/buf_add_char.c b/tests/fuzz/buf_add_char.c
index 33ca222..0f472b8 100644
--- a/tests/fuzz/buf_add_char.c
+++ b/tests/fuzz/buf_add_char.c
@@ -46,7 +46,7 @@
 
 	free(old_data);
 	free(dest);
-	ly_ctx_destroy(ctx, NULL);
+	ly_ctx_destroy(ctx);
 
 	return 0;
 }