driver-legacy/src/main/com/mongodb/DBCollection.java
public void dropIndex(final String indexName) {
        try {
            executor.execute(new DropIndexOperation(getNamespace(), indexName, getWriteConcern()), getReadConcern());
        } catch (MongoWriteConcernException e) {
            throw createWriteConcernException(e);
        }
    }
public void dropIndex(final String indexName) {
        try {
            executor.execute(new DropIndexOperation(getNamespace(), indexName, getWriteConcern()), getReadConcern());
        } catch (MongoWriteConcernException e) {
            throw createWriteConcernException(e);
        }
    }
Drops the index with the given name from this collection.

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