refactor: Remove simplify modal dialog, use direct API

The simplify feature now uses a direct API call instead of a modal dialog.
This fixes the markdown code fence issue by stripping ```html blocks
server-side. Changes:

- Delete AiSimplifyDialogForm.php (unused modal form)
- Delete ai-simplify-handler.js (unused modal JS)
- Delete ai-simplify-dialog.css (unused modal CSS)
- Remove ai_simplify_dialog library from libraries.yml
- Remove simplify_dialog route from routing.yml
- Add stripCodeFences() to SimplifyController for clean HTML output

Author: chrisns