[SPARK-3935][Core] log the number of records that has been written

There is a unused variable(count) in saveAsHadoopDataset in PairRDDFunctions.scala. The initial idea of this variable seems to count the number of records, so I am adding a log statement to log the number of records that has been written to the writer.

Author: likun <jacky.likun@huawei.com>
Author: jackylk <jacky.likun@huawei.com>

Closes #2791 from jackylk/SPARK-3935 and squashes the following commits:

a874047 [jackylk] removing the unused variable in PairRddFunctions.scala
3bf43c7 [likun] log the number of records has been written

core/src/main/scala/org/apache/spark/rdd/PairRDDFunctions.scala

@@ -1032,10 +1032,8 @@ class PairRDDFunctions[K, V](self: RDD[(K, V)])
       writer.setup(context.stageId, context.partitionId, attemptNumber)
       writer.open()
       try {
-        var count = 0
         while (iter.hasNext) {
           val record = iter.next()
-          count += 1
           writer.write(record._1.asInstanceOf[AnyRef], record._2.asInstanceOf[AnyRef])
         }
       } finally {