[SPARK-22753][SQL] Get rid of dataSource.writeAndRead

## What changes were proposed in this pull request?

As the discussion in apache#16481 and apache#18975 (comment)
Currently the BaseRelation returned by `dataSource.writeAndRead` only used in `CreateDataSourceTableAsSelect`, planForWriting and writeAndRead has some common code paths.
In this patch I removed the writeAndRead function and added the getRelation function which only use in `CreateDataSourceTableAsSelectCommand` while saving data to non-existing table.

## How was this patch tested?

Existing UT

Author: Yuanjian Li <[email protected]>

Closes apache#19941 from xuanyuanking/SPARK-22753.

Author: xuanyuanking

sql/core/src/main/scala/org/apache/spark/sql/execution/command/InsertIntoDataSourceDirCommand.scala

@@ -67,8 +67,7 @@ case class InsertIntoDataSourceDirCommand(
 
     val saveMode = if (overwrite) SaveMode.Overwrite else SaveMode.ErrorIfExists
     try {
-      sparkSession.sessionState.executePlan(dataSource.planForWriting(saveMode, query))
-      dataSource.writeAndRead(saveMode, query)
+      sparkSession.sessionState.executePlan(dataSource.planForWriting(saveMode, query)).toRdd
     } catch {
       case ex: AnalysisException =>
         logError(s"Failed to write to directory " + storage.locationUri.toString, ex)