FIX: Do not delete old chat messages if chat disabled (#21214)

Further followup to 24ec06ff85c7acbad9621092b5e50eec2ede7b83,
where I prevented other chat scheduled jobs from running if
chat was disabled. We should not be running any plugin scheduled
jobs if that plugin is disabled, it can cause unexpected
behaviour.

plugins/chat/app/jobs/scheduled/chat/delete_old_messages.rb

@@ -6,6 +6,8 @@ module Jobs
       daily at: 0.hours
 
       def execute(args = {})
+        return if !SiteSetting.chat_enabled
+
         delete_public_channel_messages
         delete_dm_channel_messages
       end