TextEditor: Use new setup pattern for PlainTextEditorFactory

Change-Id: Iad3b8574a51db3baf6db74353e58968a2f6a4315
Reviewed-by: David Schulz <david.schulz@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/plugins/texteditor/texteditorplugin.cpp

@@ -71,7 +71,6 @@ public:
     FindInCurrentFile findInCurrentFileFilter;
     FindInOpenFiles findInOpenFilesFilter;
 
-    PlainTextEditorFactory plainTextEditorFactory;
     MarkdownEditorFactory markdownEditorFactory;
     JsonEditorFactory jsonEditorFactory;
 };
@@ -117,6 +116,8 @@ void TextEditorPlugin::initialize()
     setupOutlineFactory();
     setupLineNumberFilter(); // Goto line functionality for quick open
 
+    setupPlainTextEditor();
+
     setupBookmarkManager(this);
     setupBookmarkView();
     setupBookmarkFilter();