remove unused import

Author: Hendrik Muhs

x-pack/plugin/data-frame/src/main/java/org/elasticsearch/xpack/dataframe/transforms/DataFrameIndexer.java

@@ -14,7 +14,6 @@
 import org.elasticsearch.action.search.SearchRequest;
 import org.elasticsearch.action.search.SearchResponse;
 import org.elasticsearch.action.search.ShardSearchFailure;
-import org.elasticsearch.common.Nullable;
 import org.elasticsearch.common.breaker.CircuitBreakingException;
 import org.elasticsearch.common.xcontent.XContentBuilder;
 import org.elasticsearch.search.aggregations.bucket.composite.CompositeAggregation;