Mongo should not allow removal of shard key index when the collection is not empty

Currently it is allowed to manually remove the shard key index, even if the collection is not empty. I cannot think of a situation where this is desirable and since SERVER-1506 there is not necessarily an index dedicated to the shard key.

I think it should reject removal of this index if the collection is sharded. In other words, the only way to remove the shard key collection is to drop the collection.