Currently we are unconditionally broadcasting the drop collection to all the shards also for un-sharded collections because of SERVER-33973. After 5.0 will became last LTS we can actually stop broadcasting to all the shards for the un-sharded collections.

As discussed with kaloian.manassiev we can't stop broadcasting even in 5.1 because some old drop collection run on an old binary (in which was broken) could have left garbage.