FIX: Don't show move topic for private messages for TL4 (#28871)

In TopicController, in addition to ensure_can_move_posts!, we also
checked if the topic is private message in this line:

```ruby
raise Discourse::InvalidAccess if params[:archetype] == "private_message" && !guardian.is_staff?
```

However, this was not present in `guardian.can_move_posts?`. As a result,
the frontend topic view got an incorrect serialized result, thinking
that TL4 could move the private message post. In fact, once they tried
to move it, they got the `InvalidAccess` error message.

This commit fixes that TL4 will no longer sees the "move to" option in
the "select post" panel for a private message.

app/controllers/topics_controller.rb

@@ -875,8 +875,6 @@ class TopicsController < ApplicationController
     params.permit(:chronological_order)
     params.permit(:archetype)
 
-    raise Discourse::InvalidAccess if params[:archetype] == "private_message" && !guardian.is_staff?
-
     topic = Topic.with_deleted.find_by(id: topic_id)
     guardian.ensure_can_move_posts!(topic)