use the mgo built in remove rather than a custom one

Previously we had our own function for deleting a metric batch during the Cleanup call. All we need to do is delete metrics based on a certain selection so we can just use collection.Remove() from mgo.
This needs a comment explaining why it's ok to write to the DB without going via mgo/txn. eg

    // Nothing else in the system will interact with sent metrics, and nothing needs
    // to watch them either; so in this instance it's safe to do an end run around the
    // mgo/txn package. See State.cleanupRelationSettings for a similar situation.