Remove a shard in MongoDB
Wiki answer generated from question comments by SDillon

No; at least in versions prior to 2.6. After 2.6 I am not 100% sure.

I've had this happen in development, thankfully a while ago. I dropped the collection and recovered the data to it. Here is a MongoDB email group post regarding this topic and your exact challenge.

Another mistake I made, once, was to monkey around in the config database and attempt to remove the shard info that way. I ruined that dev cluster.