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/tools/lint/cmd_clear.c b/tools/lint/cmd_clear.c
index 2a8a903..ccf6bdd 100644
--- a/tools/lint/cmd_clear.c
+++ b/tools/lint/cmd_clear.c
@@ -78,7 +78,7 @@
         goto cleanup;
     }
 
-    ly_ctx_destroy(*ctx, NULL);
+    ly_ctx_destroy(*ctx);
     *ctx = ctx_new;
 
 cleanup:
diff --git a/tools/lint/main.c b/tools/lint/main.c
index 280df8c..9f0d027 100644
--- a/tools/lint/main.c
+++ b/tools/lint/main.c
@@ -96,7 +96,7 @@
     }
 
     store_config();
-    ly_ctx_destroy(ctx, NULL);
+    ly_ctx_destroy(ctx);
 
     return 0;
 }
diff --git a/tools/lint/main_ni.c b/tools/lint/main_ni.c
index cbf0d0f..dd712ab 100644
--- a/tools/lint/main_ni.c
+++ b/tools/lint/main_ni.c
@@ -100,7 +100,7 @@
     ly_set_erase(&c->searchpaths, NULL);
 
     ly_out_free(c->out, NULL,  0);
-    ly_ctx_destroy(c->ctx, NULL);
+    ly_ctx_destroy(c->ctx);
 }
 
 static void