[web] Simplify a bit language handling
Drop `set_default_language` and `set_user_language` by considering that:
* before being bound to the user, request will have the site's default language
* cnx has the user's preferred language, hence get it back to the request when
it's bound to a connection
The first point requires some change to cubicweb's vreg faking so it doesn't
break.
That should be enough.