driver-legacy/src/main/com/mongodb/DBCollection.java
public void dropIndex(final DBObject index) {
        try {
            executor.execute(new DropIndexOperation(getNamespace(), wrap(index), getWriteConcern()), getReadConcern());
        } catch (MongoWriteConcernException e) {
            throw createWriteConcernException(e);
        }
    }
public void dropIndex(final DBObject index) {
        try {
            executor.execute(new DropIndexOperation(getNamespace(), wrap(index), getWriteConcern()), getReadConcern());
        } catch (MongoWriteConcernException e) {
            throw createWriteConcernException(e);
        }
    }
Drops an index from this collection.  The DBObject index parameter must match the specification of the index to drop, i.e. correct
key name and type must be specified.

@param index the specification of the index to drop
@throws MongoException if the index does not exist
@mongodb.driver.manual core/indexes/ Indexes