[SPARK-14145][SQL] Remove the untyped version of Dataset.groupByKey

## What changes were proposed in this pull request?
Dataset has two variants of groupByKey, one for untyped and the other for typed. It actually doesn't make as much sense to have an untyped API here, since apps that want to use untyped APIs should just use the groupBy "DataFrame" API.

## How was this patch tested?
This patch removes a method, and removes the associated tests.

Author: Reynold Xin <rxin@databricks.com>

Closes apache#11949 from rxin/SPARK-14145.

sql/core/src/main/scala/org/apache/spark/sql/Dataset.scala

@@ -1178,32 +1178,6 @@ class Dataset[T] private[sql](
       withGroupingKey.newColumns)
   }
 
-  /**
-   * :: Experimental ::
-   * Returns a [[KeyValueGroupedDataset]] where the data is grouped by the given [[Column]]
-   * expressions.
-   *
-   * @group typedrel
-   * @since 2.0.0
-   */
-  @Experimental
-  @scala.annotation.varargs
-  def groupByKey(cols: Column*): KeyValueGroupedDataset[Row, T] = {
-    val withKeyColumns = logicalPlan.output ++ cols.map(_.expr).map(UnresolvedAlias(_))
-    val withKey = Project(withKeyColumns, logicalPlan)
-    val executed = sqlContext.executePlan(withKey)
-
-    val dataAttributes = executed.analyzed.output.dropRight(cols.size)
-    val keyAttributes = executed.analyzed.output.takeRight(cols.size)
-
-    new KeyValueGroupedDataset(
-      RowEncoder(keyAttributes.toStructType),
-      encoderFor[T],
-      executed,
-      dataAttributes,
-      keyAttributes)
-  }
-
   /**
    * :: Experimental ::
    * (Java-specific)

sql/core/src/test/java/test/org/apache/spark/sql/JavaDatasetSuite.java

@@ -245,29 +245,6 @@ public Iterator<String> call(Integer key, Iterator<String> left, Iterator<Intege
     Assert.assertEquals(asSet("1a#2", "3foobar#6", "5#10"), toSet(cogrouped.collectAsList()));
   }
 
-  @Test
-  public void testGroupByColumn() {
-    List<String> data = Arrays.asList("a", "foo", "bar");
-    Dataset<String> ds = context.createDataset(data, Encoders.STRING());
-    KeyValueGroupedDataset<Integer, String> grouped =
-      ds.groupByKey(length(col("value"))).keyAs(Encoders.INT());
-
-    Dataset<String> mapped = grouped.mapGroups(
-      new MapGroupsFunction<Integer, String, String>() {
-        @Override
-        public String call(Integer key, Iterator<String> data) throws Exception {
-          StringBuilder sb = new StringBuilder(key.toString());
-          while (data.hasNext()) {
-            sb.append(data.next());
-          }
-          return sb.toString();
-        }
-      },
-      Encoders.STRING());
-
-    Assert.assertEquals(asSet("1a", "3foobar"), toSet(mapped.collectAsList()));
-  }
-
   @Test
   public void testSelect() {
     List<Integer> data = Arrays.asList(2, 6);

sql/core/src/test/scala/org/apache/spark/sql/DatasetCacheSuite.scala

@@ -63,7 +63,7 @@ class DatasetCacheSuite extends QueryTest with SharedSQLContext {
 
   test("persist and then groupBy columns asKey, map") {
     val ds = Seq(("a", 10), ("a", 20), ("b", 1), ("b", 2), ("c", 1)).toDS()
-    val grouped = ds.groupByKey($"_1").keyAs[String]
+    val grouped = ds.groupByKey(_._1)
     val agged = grouped.mapGroups { case (g, iter) => (g, iter.map(_._2).sum) }
     agged.persist()
 

sql/core/src/test/scala/org/apache/spark/sql/DatasetSuite.scala

@@ -322,55 +322,6 @@ class DatasetSuite extends QueryTest with SharedSQLContext {
     )
   }
 
-  test("groupBy columns, map") {
-    val ds = Seq(("a", 10), ("a", 20), ("b", 1), ("b", 2), ("c", 1)).toDS()
-    val grouped = ds.groupByKey($"_1")
-    val agged = grouped.mapGroups { case (g, iter) => (g.getString(0), iter.map(_._2).sum) }
-
-    checkDataset(
-      agged,
-      ("a", 30), ("b", 3), ("c", 1))
-  }
-
-  test("groupBy columns, count") {
-    val ds = Seq("a" -> 1, "b" -> 1, "a" -> 2).toDS()
-    val count = ds.groupByKey($"_1").count()
-
-    checkDataset(
-      count,
-      (Row("a"), 2L), (Row("b"), 1L))
-  }
-
-  test("groupBy columns asKey, map") {
-    val ds = Seq(("a", 10), ("a", 20), ("b", 1), ("b", 2), ("c", 1)).toDS()
-    val grouped = ds.groupByKey($"_1").keyAs[String]
-    val agged = grouped.mapGroups { case (g, iter) => (g, iter.map(_._2).sum) }
-
-    checkDataset(
-      agged,
-      ("a", 30), ("b", 3), ("c", 1))
-  }
-
-  test("groupBy columns asKey tuple, map") {
-    val ds = Seq(("a", 10), ("a", 20), ("b", 1), ("b", 2), ("c", 1)).toDS()
-    val grouped = ds.groupByKey($"_1", lit(1)).keyAs[(String, Int)]
-    val agged = grouped.mapGroups { case (g, iter) => (g, iter.map(_._2).sum) }
-
-    checkDataset(
-      agged,
-      (("a", 1), 30), (("b", 1), 3), (("c", 1), 1))
-  }
-
-  test("groupBy columns asKey class, map") {
-    val ds = Seq(("a", 10), ("a", 20), ("b", 1), ("b", 2), ("c", 1)).toDS()
-    val grouped = ds.groupByKey($"_1".as("a"), lit(1).as("b")).keyAs[ClassData]
-    val agged = grouped.mapGroups { case (g, iter) => (g, iter.map(_._2).sum) }
-
-    checkDataset(
-      agged,
-      (ClassData("a", 1), 30), (ClassData("b", 1), 3), (ClassData("c", 1), 1))
-  }
-
   test("typed aggregation: expr") {
     val ds = Seq(("a", 10), ("a", 20), ("b", 1), ("b", 2), ("c", 1)).toDS()