Get rid of no longer needed target_usernames warning log

lib/topic_creator.rb

@@ -171,8 +171,6 @@ class TopicCreator
     topic.subtype = TopicSubtype.user_to_user unless topic.subtype
 
     unless @opts[:target_usernames].present? || @opts[:target_emails].present? || @opts[:target_group_names].present?
-      Rails.logger.warn("Topic PM cannot be created without recipients! opts: #{@opts.inspect}")
-
       rollback_with!(topic, :no_user_selected)
     end