DEV: Use guardian user for can_chat? (#19418)

Instead of passing `user` to `guardian.can_chat?`, we
can just use the inner `@user` that is part of the guardian
instance already to determine whether that user can chat,
since this is how it works for all other usages of guardian
even within chat.

plugins/chat/app/controllers/api_controller.rb

@@ -8,6 +8,6 @@ class Chat::Api < Chat::ChatBaseController
 
   def ensure_can_chat
     raise Discourse::NotFound unless SiteSetting.chat_enabled
-    guardian.ensure_can_chat!(current_user)
+    guardian.ensure_can_chat!
   end
 end