[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](caea152145) 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 <lwlin7@gmail.com>

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

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

@@ -1,26 +0,0 @@
-/*
- * Licensed to the Apache Software Foundation (ASF) under one or more
- * contributor license agreements.  See the NOTICE file distributed with
- * this work for additional information regarding copyright ownership.
- * The ASF licenses this file to You under the Apache License, Version 2.0
- * (the "License"); you may not use this file except in compliance with
- * the License.  You may obtain a copy of the License at
- *
- *    http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-package org.apache.spark.sql.execution.streaming
-
-import org.apache.spark.sql.DataFrame
-
-/**
- * Used to pass a batch of data through a streaming query execution along with an indication
- * of progress in the stream.
- */
-class Batch(val end: Offset, val data: DataFrame)

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