[IMP] website: remove useless lang redirect on /robots.txt

Same as what was done:
- in 2019 for favicon.ico at [1].
- in 2018 for sitemap.xml at [2].
- in 2015 for many controllers at [3].

Note that it's not about crawlers/robots, because those won't go through
the lang redirect (see `is_a_bot()`).

It's just to avoid a redirect which has no meaning/sense when requested
by real users. /robots.txt is not language dependent.

[1]: https://github.com/odoo/odoo/commit/94bcbc92e5e5a6fd3de7267e3c01f8c11fb045f4
[2]: https://github.com/odoo/odoo/commit/708ad186d52aff9270ea73888326370ada2fbc71
[3]: https://github.com/odoo/odoo/commit/a696913364ffc4d5f1ce0675bc9be82f84f3ff93

closes odoo/odoo#145018

Signed-off-by: Jérémy Kersten <jke@odoo.com>

addons/website/controllers/main.py

@@ -216,7 +216,7 @@ class Website(Home):
         fields = country.get_address_fields()
         return dict(fields=fields, states=[(st.id, st.name, st.code) for st in country.state_ids], phone_code=country.phone_code)
 
-    @http.route(['/robots.txt'], type='http', auth="public", website=True, sitemap=False)
+    @http.route(['/robots.txt'], type='http', auth="public", website=True, multilang=False, sitemap=False)
     def robots(self, **kwargs):
         return request.render('website.robots', {'url_root': request.httprequest.url_root}, mimetype='text/plain')