Remove BulkDownloader operator (#3161)

This operator's design requires some more discussion. The current issues
are:
1. it is accessing the workflow context (i.e., workflowId and
executionId), which may not be permitted in the future.
2. It is directly interacting with the local file system.

For the short term, we will drop the support for such as downloader. We
can discuss how to support it in the future when needed.

core/workflow-operator/src/main/scala/edu/uci/ics/amber/operator/LogicalOp.scala

@@ -11,7 +11,6 @@ import edu.uci.ics.amber.operator.cartesianProduct.CartesianProductOpDesc
 import edu.uci.ics.amber.operator.dictionary.DictionaryMatcherOpDesc
 import edu.uci.ics.amber.operator.difference.DifferenceOpDesc
 import edu.uci.ics.amber.operator.distinct.DistinctOpDesc
-import edu.uci.ics.amber.operator.download.BulkDownloaderOpDesc
 import edu.uci.ics.amber.operator.dummy.DummyOpDesc
 import edu.uci.ics.amber.operator.filter.SpecializedFilterOpDesc
 import edu.uci.ics.amber.operator.hashJoin.HashJoinOpDesc
@@ -202,7 +201,6 @@ trait StateTransferFunc
     new Type(value = classOf[RedditSearchSourceOpDesc], name = "RedditSearch"),
     new Type(value = classOf[PythonLambdaFunctionOpDesc], name = "PythonLambdaFunction"),
     new Type(value = classOf[PythonTableReducerOpDesc], name = "PythonTableReducer"),
-    new Type(value = classOf[BulkDownloaderOpDesc], name = "BulkDownloader"),
     new Type(value = classOf[URLFetcherOpDesc], name = "URLFetcher"),
     new Type(value = classOf[CartesianProductOpDesc], name = "CartesianProduct"),
     new Type(value = classOf[FilledAreaPlotOpDesc], name = "FilledAreaPlot"),