[SPARK-35526][CORE][SQL][ML][MLLIB] Re-Cleanup procedure syntax is deprecated compilation warning in Scala 2.13

### What changes were proposed in this pull request?
After SPARK-29291 and SPARK-33352, there are still some compilation warnings about `procedure syntax is deprecated` as follows:

```
[WARNING] [Warn] /spark/core/src/main/scala/org/apache/spark/MapOutputTracker.scala:723: [deprecation   | origin= | version=2.13.0] procedure syntax is deprecated: instead, add `: Unit =` to explicitly declare `registerMergeResult`'s return type
[WARNING] [Warn] /spark/core/src/main/scala/org/apache/spark/MapOutputTracker.scala:748: [deprecation   | origin= | version=2.13.0] procedure syntax is deprecated: instead, add `: Unit =` to explicitly declare `unregisterMergeResult`'s return type
[WARNING] [Warn] /spark/core/src/test/scala/org/apache/spark/util/collection/ExternalAppendOnlyMapSuite.scala:223: [deprecation   | origin= | version=2.13.0] procedure syntax is deprecated: instead, add `: Unit =` to explicitly declare `testSimpleSpillingForAllCodecs`'s return type
[WARNING] [Warn] /spark/mllib-local/src/test/scala/org/apache/spark/ml/linalg/BLASBenchmark.scala:53: [deprecation   | origin= | version=2.13.0] procedure syntax is deprecated: instead, add `: Unit =` to explicitly declare `runBLASBenchmark`'s return type
[WARNING] [Warn] /spark/sql/core/src/main/scala/org/apache/spark/sql/execution/command/DataWritingCommand.scala:110: [deprecation   | origin= | version=2.13.0] procedure syntax is deprecated: instead, add `: Unit =` to explicitly declare `assertEmptyRootPath`'s return type
[WARNING] [Warn] /spark/sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/SQLQuerySuite.scala:602: [deprecation   | origin= | version=2.13.0] procedure syntax is deprecated: instead, add `: Unit =` to explicitly declare `executeCTASWithNonEmptyLocation`'s return type
```

So the main change of this pr is cleanup these compilation warnings.

### Why are the changes needed?
Eliminate compilation warnings in Scala 2.13 and this change should be compatible with Scala 2.12

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

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

Closes #32669 from LuciferYang/re-clean-procedure-syntax.

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

core/src/main/scala/org/apache/spark/MapOutputTracker.scala

@@ -720,7 +720,7 @@ private[spark] class MapOutputTrackerMaster(
     }
   }
 
-  def registerMergeResult(shuffleId: Int, reduceId: Int, status: MergeStatus) {
+  def registerMergeResult(shuffleId: Int, reduceId: Int, status: MergeStatus): Unit = {
     shuffleStatuses(shuffleId).addMergeResult(reduceId, status)
   }
 
@@ -745,7 +745,7 @@ private[spark] class MapOutputTrackerMaster(
     shuffleId: Int,
     reduceId: Int,
     bmAddress: BlockManagerId,
-    mapId: Option[Int] = None) {
+    mapId: Option[Int] = None): Unit = {
     shuffleStatuses.get(shuffleId) match {
       case Some(shuffleStatus) =>
         val mergeStatus = shuffleStatus.mergeStatuses(reduceId)

core/src/test/scala/org/apache/spark/util/collection/ExternalAppendOnlyMapSuite.scala

@@ -220,7 +220,7 @@ class ExternalAppendOnlyMapSuite extends SparkFunSuite
     testSimpleSpilling()
   }
 
-  private def testSimpleSpillingForAllCodecs(encrypt: Boolean) {
+  private def testSimpleSpillingForAllCodecs(encrypt: Boolean): Unit = {
     // Keep track of which compression codec we're using to report in test failure messages
     var lastCompressionCodec: Option[String] = None
     try {

mllib-local/src/test/scala/org/apache/spark/ml/linalg/BLASBenchmark.scala

@@ -50,7 +50,7 @@ object BLASBenchmark extends BenchmarkBase {
     println("nativeBLAS = " + nativeBLAS.getClass.getName)
     // scalastyle:on println
 
-    def runBLASBenchmark(name: String, n: Int)(bench: NetlibBLAS => Unit) {
+    def runBLASBenchmark(name: String, n: Int)(bench: NetlibBLAS => Unit): Unit = {
       val benchmark = new Benchmark(name, n, iters,
                                     warmupTime = 30.seconds,
                                     minTime = 30.seconds,

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

@@ -107,7 +107,7 @@ object DataWritingCommand {
    * @param saveMode  Save mode of the table.
    * @param hadoopConf Configuration.
    */
-  def assertEmptyRootPath(tablePath: URI, saveMode: SaveMode, hadoopConf: Configuration) {
+  def assertEmptyRootPath(tablePath: URI, saveMode: SaveMode, hadoopConf: Configuration): Unit = {
     if (saveMode == SaveMode.ErrorIfExists && !SQLConf.get.allowNonEmptyLocationInCTAS) {
       val filePath = new org.apache.hadoop.fs.Path(tablePath)
       val fs = filePath.getFileSystem(hadoopConf)

sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/SQLQuerySuite.scala

@@ -599,7 +599,7 @@ abstract class SQLQuerySuiteBase extends QueryTest with SQLTestUtils with TestHi
   }
 
   test("SPARK-28551: CTAS Hive Table should be with non-existent or empty location") {
-    def executeCTASWithNonEmptyLocation(tempLocation: String) {
+    def executeCTASWithNonEmptyLocation(tempLocation: String): Unit = {
       sql(s"CREATE TABLE ctas1(id string) stored as rcfile LOCATION '$tempLocation/ctas1'")
       sql("INSERT INTO TABLE ctas1 SELECT 'A' ")
       sql(s"""CREATE TABLE ctas_with_existing_location stored as rcfile LOCATION