[SPARK-21357][DSTREAMS] FileInputDStream not remove out of date RDD

## What changes were proposed in this pull request?

```DStreams
         class FileInputDStream

 [line 162]   protected[streaming] override def clearMetadata(time: Time) {
    batchTimeToSelectedFiles.synchronized {
      val oldFiles = batchTimeToSelectedFiles.filter(_._1 < (time - rememberDuration))
      batchTimeToSelectedFiles --= oldFiles.keys

```
The above code does not remove the old generatedRDDs. "super.clearMetadata(time)" was added to the beginning of clearMetadata to remove the old generatedRDDs.

## How was this patch tested?

At the end of clearMetadata, the testing code (print the number of generatedRDDs) was added to check the old RDDS were removed manually.

Author: shaofei007 <1427357147@qq.com>
Author: Fei Shao <1427357147@qq.com>

Closes #18718 from shaofei007/master.

streaming/src/main/scala/org/apache/spark/streaming/dstream/FileInputDStream.scala

@@ -164,6 +164,7 @@ class FileInputDStream[K, V, F <: NewInputFormat[K, V]](
 
   /** Clear the old time-to-files mappings along with old RDDs */
   protected[streaming] override def clearMetadata(time: Time) {
+    super.clearMetadata(time)
     batchTimeToSelectedFiles.synchronized {
       val oldFiles = batchTimeToSelectedFiles.filter(_._1 < (time - rememberDuration))
       batchTimeToSelectedFiles --= oldFiles.keys