TextEditor: Use new setup pattern for markdown and json editor

And remove the now-empty plugin pimpl.

Change-Id: Ie00b747a4a41bc6efc84ea43db81a71e8364df3d
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

src/plugins/texteditor/jsoneditor.h

@@ -3,14 +3,8 @@
 
 #pragma once
 
-#include "texteditor.h"
-
 namespace TextEditor::Internal {
 
-class JsonEditorFactory final : public TextEditorFactory
-{
-public:
-    JsonEditorFactory();
-};
+void setupJsonEditor();
 
 } // TextEditor::Internal