The mongodb exercise is returning the _id field

According to Brian, the following line
https://github.com/btholt/db-samples/blob/c8936d2b1caf4a88982ce03ba37477c2376e5aab/mongodb/server.js#L26
is used to remove the `_id` field from the query response, but the query is returning the `_id` field.


Start at server.js line 26 in the linked db-samples revision and inspect the MongoDB query projection used by the exercise. Run the exercise and compare its response with the expected result; done means the returned documents no longer include the _id field.

Written by the indexing model from the issue text.