[MINOR][CORE] Fix a HadoopRDD log message and remove unused imports in rdd files.

## What changes were proposed in this pull request?

This PR fixes the following typos in log message and comments of `HadoopRDD.scala`. Also, this removes unused imports.
```scala
-      logWarning("Caching NewHadoopRDDs as deserialized objects usually leads to undesired" +
+      logWarning("Caching HadoopRDDs as deserialized objects usually leads to undesired" +
...
-      // since its not removed yet
+      // since it's not removed yet
```

## How was this patch tested?

Manual.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #13294 from dongjoon-hyun/minor_rdd_fix_log_message.

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

@@ -17,7 +17,7 @@
 
 package org.apache.spark.rdd
 
-import org.apache.hadoop.conf.{ Configurable, Configuration }
+import org.apache.hadoop.conf.{Configurable, Configuration}
 import org.apache.hadoop.io.Writable
 import org.apache.hadoop.mapreduce._
 import org.apache.hadoop.mapreduce.task.JobContextImpl

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

@@ -43,7 +43,6 @@ import org.apache.spark._
 import org.apache.spark.annotation.DeveloperApi
 import org.apache.spark.broadcast.Broadcast
 import org.apache.spark.deploy.SparkHadoopUtil
-import org.apache.spark.executor.DataReadMethod
 import org.apache.spark.internal.Logging
 import org.apache.spark.rdd.HadoopRDD.HadoopMapPartitionsWithSplitRDD
 import org.apache.spark.scheduler.{HDFSCacheTaskLocation, HostTaskLocation}
@@ -70,7 +69,7 @@ private[spark] class HadoopPartition(rddId: Int, override val index: Int, s: Inp
     val envVars: Map[String, String] = if (inputSplit.value.isInstanceOf[FileSplit]) {
       val is: FileSplit = inputSplit.value.asInstanceOf[FileSplit]
       // map_input_file is deprecated in favor of mapreduce_map_input_file but set both
-      // since its not removed yet
+      // since it's not removed yet
       Map("map_input_file" -> is.getPath().toString(),
         "mapreduce_map_input_file" -> is.getPath().toString())
     } else {
@@ -335,7 +334,7 @@ class HadoopRDD[K, V](
 
   override def persist(storageLevel: StorageLevel): this.type = {
     if (storageLevel.deserialized) {
-      logWarning("Caching NewHadoopRDDs as deserialized objects usually leads to undesired" +
+      logWarning("Caching HadoopRDDs as deserialized objects usually leads to undesired" +
         " behavior because Hadoop's RecordReader reuses the same Writable object for all records." +
         " Use a map transformation to make copies of the records.")
     }

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

@@ -32,7 +32,6 @@ import org.apache.hadoop.mapreduce.task.{JobContextImpl, TaskAttemptContextImpl}
 import org.apache.spark._
 import org.apache.spark.annotation.DeveloperApi
 import org.apache.spark.deploy.SparkHadoopUtil
-import org.apache.spark.executor.DataReadMethod
 import org.apache.spark.internal.Logging
 import org.apache.spark.rdd.NewHadoopRDD.NewHadoopMapPartitionsWithSplitRDD
 import org.apache.spark.storage.StorageLevel

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

@@ -40,7 +40,7 @@ import org.apache.spark._
 import org.apache.spark.Partitioner.defaultPartitioner
 import org.apache.spark.annotation.Experimental
 import org.apache.spark.deploy.SparkHadoopUtil
-import org.apache.spark.executor.{DataWriteMethod, OutputMetrics}
+import org.apache.spark.executor.OutputMetrics
 import org.apache.spark.internal.Logging
 import org.apache.spark.partial.{BoundedDouble, PartialResult}
 import org.apache.spark.serializer.Serializer

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

@@ -31,7 +31,6 @@ import scala.collection.Map
 import scala.collection.mutable.ArrayBuffer
 import scala.io.Source
 import scala.reflect.ClassTag
-import scala.util.control.NonFatal
 
 import org.apache.spark.{Partition, SparkEnv, TaskContext}
 import org.apache.spark.util.Utils