### 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>
09d039da56