MM-30865 Invalid Participants handling on post delete in thread (#16353)

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

store/sqlstore/post_store.go

@@ -1937,7 +1937,6 @@ func (s *SqlPostStore) cleanupThreads(postId, rootId, userId string, permanent b
 		}
 		if thread != nil {
 			thread.ReplyCount -= 1
-			thread.Participants = thread.Participants.Remove(userId)
 			if _, err = s.Thread().Update(thread); err != nil {
 				return errors.Wrap(err, "failed to update thread")
 			}