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/context.h b/src/context.h
index cda1212..8f0d470 100644
--- a/src/context.h
+++ b/src/context.h
@@ -593,11 +593,12 @@
  * All instance data are supposed to be freed before destroying the context.
  * Data models are destroyed automatically as part of ::ly_ctx_destroy() call.
  *
+ * Note that the data stored by user into the ::lysc_node.priv pointer are kept
+ * untouched and the caller is responsible for freeing this private data.
+ *
  * @param[in] ctx libyang context to destroy
- * @param[in] private_destructor Optional destructor function for private objects assigned
- * to the schema nodes' priv pointer. If NULL, the private objects are not freed by libyang.
  */
-void ly_ctx_destroy(struct ly_ctx *ctx, void (*private_destructor)(const struct lysc_node *node, void *priv));
+void ly_ctx_destroy(struct ly_ctx *ctx);
 
 /** @} context */