DEV: Remove experimental site setting for chat threads (#22720)

We are removing the experimental site setting. Admins can now decide on a per channel basis to enable/disable threading. It's disabled by default.

plugins/chat/lib/chat/channel_fetcher.rb

@@ -245,8 +245,7 @@ module Chat
         channel_ids: channel_ids,
         guardian: guardian,
         include_missing_memberships: true,
-        include_threads:
-          SiteSetting.enable_experimental_chat_threaded_discussions && include_threads,
+        include_threads: include_threads,
       ).report
     end
 

plugins/chat/lib/chat/message_mover.rb

@@ -25,8 +25,7 @@
 # into a new channel, they end up as just a flat series of messages that are
 # not in a chain. If the original message of a thread and N other messages
 # in that thread, then any messages left behind just get placed into a new
-# thread. Message moving will be disabled in the thread UI while
-# enable_experimental_chat_threaded_discussions is present, its too complicated
+# thread. Message moving will be disabled in the thread UI, its too complicated
 # to have end users reason about for now, and we may want a standalone
 # "Move Thread" UI later on.
 module Chat

plugins/chat/lib/discourse_dev/thread.rb

@@ -13,10 +13,6 @@ module DiscourseDev
     end
 
     def data
-      if !SiteSetting.enable_experimental_chat_threaded_discussions
-        raise "You need to enable_experimental_chat_threaded_discussions to run this task"
-      end
-
       channel = ::Chat::Channel.find(@channel_id)
       return if !channel