[SPARK-14874][SQL][STREAMING] Remove the obsolete Batch representation

## What changes were proposed in this pull request?

The `Batch` class, which had been used to indicate progress in a stream, was abandoned by [[SPARK-13985][SQL] Deterministic batches with ids](apache@caea152) and then became useless.

This patch:
- removes the `Batch` class
- ~~does some related renaming~~ (update: this has been reverted)
- fixes some related comments

## How was this patch tested?

N/A

Author: Liwei Lin <[email protected]>

Closes apache#12638 from lw-lin/remove-batch.

Author: lw-lin

sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/Batch.scala

@@ -88,7 +88,7 @@ class FileStreamSource(
   }
 
   /**
-   * Returns the next batch of data that is available after `start`, if any is available.
+   * Returns the data that is between the offsets (`start`, `end`].
    */
   override def getBatch(start: Option[Offset], end: Offset): DataFrame = {
     val startId = start.map(_.asInstanceOf[LongOffset].offset).getOrElse(-1L)

sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/FileStreamSource.scala

@@ -27,7 +27,7 @@ import org.apache.spark.sql.DataFrame
 trait Sink {
 
   /**
-   * Adds a batch of data to this sink.  The data for a given `batchId` is deterministic and if
+   * Adds a batch of data to this sink. The data for a given `batchId` is deterministic and if
    * this method is called more than once with the same batchId (which will happen in the case of
    * failures), then `data` should only be added once.
    */

sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/Sink.scala

@@ -34,7 +34,7 @@ trait Source  {
   def getOffset: Option[Offset]
 
   /**
-   * Returns the data that is between the offsets (`start`, `end`].  When `start` is `None` then
+   * Returns the data that is between the offsets (`start`, `end`]. When `start` is `None` then
    * the batch should begin with the first available record. This method must always return the
    * same data for a particular `start` and `end` pair.
    */

sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/Source.scala

@@ -91,7 +91,7 @@ case class MemoryStream[A : Encoder](id: Int, sqlContext: SQLContext)
   }
 
   /**
-   * Returns the next batch of data that is available after `start`, if any is available.
+   * Returns the data that is between the offsets (`start`, `end`].
    */
   override def getBatch(start: Option[Offset], end: Offset): DataFrame = {
     val startOrdinal =