[SPARK-33283][CORE] Remove useless externalBlockStoreSize from RDDInfo

### What changes were proposed in this pull request?
"external block store" API was removed after SPARK-12667,  `externalBlockStoreSize` in `RDDInfo` looks like always 0 and useless. So this pr just to remove this useless variable.

### Why are the changes needed?
remove useless variable.

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
Pass the Jenkins or GitHub Action

Closes #30179 from LuciferYang/SPARK-12667-FOLLOWUP.

Authored-by: yangjie01 <yangjie01@baidu.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

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

@@ -1919,9 +1919,8 @@ abstract class RDD[T: ClassTag](
 
       val persistence = if (storageLevel != StorageLevel.NONE) storageLevel.description else ""
       val storageInfo = rdd.context.getRDDStorageInfo(_.id == rdd.id).map(info =>
-        "    CachedPartitions: %d; MemorySize: %s; ExternalBlockStoreSize: %s; DiskSize: %s".format(
-          info.numCachedPartitions, bytesToString(info.memSize),
-          bytesToString(info.externalBlockStoreSize), bytesToString(info.diskSize)))
+        "    CachedPartitions: %d; MemorySize: %s; DiskSize: %s".format(
+          info.numCachedPartitions, bytesToString(info.memSize), bytesToString(info.diskSize)))
 
       s"$rdd [$persistence]" +: storageInfo
     }

core/src/main/scala/org/apache/spark/storage/BlockManagerMasterEndpoint.scala

@@ -737,7 +737,6 @@ private[spark] class BlockManagerInfo(
     if (storageLevel.isValid) {
       /* isValid means it is either stored in-memory or on-disk.
        * The memSize here indicates the data size in or dropped from memory,
-       * externalBlockStoreSize here indicates the data size in or dropped from externalBlockStore,
        * and the diskSize here indicates the data size in or dropped to disk.
        * They can be both larger than 0, when a block is dropped from memory to disk.
        * Therefore, a safe way to set BlockStatus is to set its info in accurate modes. */

core/src/main/scala/org/apache/spark/storage/RDDInfo.scala

@@ -38,7 +38,6 @@ class RDDInfo(
   var numCachedPartitions = 0
   var memSize = 0L
   var diskSize = 0L
-  var externalBlockStoreSize = 0L
 
   def isCached: Boolean = (memSize + diskSize > 0) && numCachedPartitions > 0