Fix memory leak in richtext sample.

Destroy the children of wxRichTextFieldTypeCompositeTest when updating it
instead of just clearing the list and leaking the objects in it.

Closes #14324.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78037 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

samples/richtext/richtext.cpp

@@ -127,7 +127,7 @@ public:
             attr.SetLineSpacing(10);
             obj->SetAttributes(attr);
         }
-        obj->GetChildren().Clear();
+        obj->DeleteChildren();
         wxRichTextParagraph* para = new wxRichTextParagraph;
         wxRichTextPlainText* text = new wxRichTextPlainText(GetLabel());
         para->AppendChild(text);