Allow dropping the _id index when you have a hashed _id index

If hashed indexes can function as unique indexes, then it's not really necessary to maintain both an index on {_id : 1} and one on {_id : "hashed"}. There's an obvious cost to maintaining both indexes (unless you need the original one for range queries). Thus if/when SERVER-5878 is implemented, the user should be allowed to drop the default {_id : 1} index.