[FIX] website: remove unused 'deletable' key

This was introduced with [1] 8 years ago (refactored with [2] 5 years
ago).
It was no more used since rencently with [3] (fontend > backend task),
but the key was not removed.

[1]: https://github.com/odoo/odoo/commit/905dd42b87ff645321bab7f4f3088af848315763
[2]: https://github.com/odoo/odoo/commit/4ecbacaf59576a22ff45615a5aa5c67244e4fb93
[3]: https://github.com/odoo/odoo/commit/31cc10b91dc7762e23b4bde9b945be0c4ce3fe3b

Part-of: odoo/odoo#98200

addons/website/models/ir_http.py

@@ -271,7 +271,6 @@ class Http(models.AbstractModel):
         if page and (request.website.is_publisher() or page.is_visible):
             _, ext = os.path.splitext(req_page)
             response = request.render(page.view_id.id, {
-                'deletable': True,
                 'main_object': page,
             }, mimetype=_guess_mimetype(ext))
             return response