There are better ways to prevent downvote brigades than removing dislikes altogether. Not to mention that keeping likes around doesn't solve anything, as it can be exploited in the same way by upvote brigades! As long as you don't start to actively detect fake accounts and don't limit the number of upvotes / downvotes per day a person can give, it will always be possible to game the upvote metric by a small group of people / bots.