[SPARK-8908] [SQL] Add () to distinct definition in dataframe

Adding `()` to the definition of `distinct` in DataFrame allows distinct to be called with parentheses, which is consistent with `dropDuplicates`.

Author: Cheolsoo Park <cheolsoop@netflix.com>

Closes #7298 from piaozhexiu/SPARK-8908 and squashes the following commits:

7f0d923 [Cheolsoo Park] Add () to distinct definition in dataframe

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

@@ -1415,7 +1415,7 @@ class DataFrame private[sql](
    * @group dfops
    * @since 1.3.0
    */
-  override def distinct: DataFrame = dropDuplicates()
+  override def distinct(): DataFrame = dropDuplicates()
 
   /**
    * @group basic