FIX: Strip discourse-logged-in header during force_anonymous! (#14533)

When the anonymous cache forces users into anonymous mode, it strips the cookies from their request. However, the discourse-logged-in header from the JS client remained.

When the discourse-logged-in header is present without any valid auth_token, the current_user_provider [marks the request as ['logged out'](https://github.com/discourse/discourse/blob/dbbfad7ed07c47674f9dee4ac7021ca51cc04e2e/lib/auth/default_current_user_provider.rb#L125-L125), and a [discourse-logged-out header is returned to the client](https://github.com/discourse/discourse/blob/dbbfad7ed07c47674f9dee4ac7021ca51cc04e2e/lib/middleware/request_tracker.rb#L209-L211). This causes the JS app to [popup a "you were logged out" modal](https://github.com/discourse/discourse/blob/dbbfad7ed07c47674f9dee4ac7021ca51cc04e2e/app/assets/javascripts/discourse/app/components/d-document.js#L29-L29), which is very disruptive.

This commit strips the discourse-logged-in header from the request at the same time as the auth cookie.

lib/middleware/anonymous_cache.rb

@@ -171,6 +171,7 @@ module Middleware
       def force_anonymous!
         @env[Auth::DefaultCurrentUserProvider::USER_API_KEY] = nil
         @env['HTTP_COOKIE'] = nil
+        @env['HTTP_DISCOURSE_LOGGED_IN'] = nil
         @env['rack.request.cookie.hash'] = {}
         @env['rack.request.cookie.string'] = ''
         @env['_bypass_cache'] = nil