[SPARK-23020][CORE][FOLLOWUP] Fix Java style check issues.

## What changes were proposed in this pull request?

This is a follow-up of #20297 which broke lint-java checks.
This pr fixes the lint-java issues.

```
[ERROR] src/test/java/org/apache/spark/launcher/BaseSuite.java:[21,8] (imports) UnusedImports: Unused import - java.util.concurrent.TimeUnit.
[ERROR] src/test/java/org/apache/spark/launcher/SparkLauncherSuite.java:[27,8] (imports) UnusedImports: Unused import - java.util.concurrent.TimeUnit.
```

## How was this patch tested?

Checked manually in my local environment.

Author: Takuya UESHIN <[email protected]>

Closes #20376 from ueshin/issues/SPARK-23020/fup1.

Author: ueshin

core/src/test/java/org/apache/spark/launcher/SparkLauncherSuite.java

@@ -24,7 +24,6 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Properties;
-import java.util.concurrent.TimeUnit;
 
 import org.junit.Test;
 import static org.junit.Assert.*;

launcher/src/test/java/org/apache/spark/launcher/BaseSuite.java

@@ -18,7 +18,6 @@
 package org.apache.spark.launcher;
 
 import java.time.Duration;
-import java.util.concurrent.TimeUnit;
 
 import org.junit.After;
 import org.slf4j.bridge.SLF4JBridgeHandler;