[SPARK-18030][TESTS] Fix flaky FileStreamSourceSuite by not deleting the files

## What changes were proposed in this pull request?

The test `when schema inference is turned on, should read partition data` should not delete files because the source maybe is listing files. This PR just removes the delete actions since they are not necessary.

## How was this patch tested?

Jenkins

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #15699 from zsxwing/SPARK-18030.

sql/core/src/test/scala/org/apache/spark/sql/streaming/FileStreamSourceSuite.scala

@@ -102,12 +102,6 @@ class FileStreamSourceTest extends StreamTest with SharedSQLContext with Private
     }
   }
 
-  case class DeleteFile(file: File) extends ExternalAction {
-    def runAction(): Unit = {
-      Utils.deleteRecursively(file)
-    }
-  }
-
   /** Use `format` and `path` to create FileStreamSource via DataFrameReader */
   def createFileStream(
       format: String,
@@ -697,10 +691,6 @@ class FileStreamSourceSuite extends FileStreamSourceTest {
           AddTextFileData("{'value': 'keep5'}", partitionBarSubDir, tmp),
           CheckAnswer(("keep2", "foo"), ("keep3", "foo"), ("keep4", "bar"), ("keep5", "bar")),
 
-          // Delete the two partition dirs
-          DeleteFile(partitionFooSubDir),
-          DeleteFile(partitionBarSubDir),
-
           AddTextFileData("{'value': 'keep6'}", partitionBarSubDir, tmp),
           CheckAnswer(("keep2", "foo"), ("keep3", "foo"), ("keep4", "bar"), ("keep5", "bar"),
             ("keep6", "bar"))