[SPARK-30159][SQL][FOLLOWUP] Fix lint-java via removing unnecessary imports

### What changes were proposed in this pull request?

This patch fixes the Java code style violations in SPARK-30159 (#26788) which are caught by lint-java (Github Action caught it and I can reproduce it locally). Looks like Jenkins build may have different policy on checking Java style check or less accurate.

### Why are the changes needed?

Java linter starts complaining.

### Does this PR introduce any user-facing change?

No.

### How was this patch tested?

lint-java passed locally

This closes #26819

Closes #26818 from HeartSaVioR/SPARK-30159-FOLLOWUP.

Authored-by: Jungtaek Lim (HeartSaVioR) <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: HeartSaVioR

external/avro/src/test/java/org/apache/spark/sql/avro/JavaAvroFunctionsSuite.java

@@ -18,7 +18,6 @@
 package org.apache.spark.sql.avro;
 
 import org.junit.After;
-import org.junit.Assert;
 import org.junit.Before;
 import org.junit.Test;
 

sql/core/src/test/java/test/org/apache/spark/sql/JavaSaveLoadSuite.java

@@ -25,7 +25,6 @@
 import java.util.Map;
 
 import org.junit.After;
-import org.junit.Assert;
 import org.junit.Before;
 import org.junit.Test;
 

sql/hive/src/test/java/org/apache/spark/sql/hive/JavaDataFrameSuite.java

@@ -22,7 +22,6 @@
 import java.util.List;
 
 import org.junit.After;
-import org.junit.Assert;
 import org.junit.Before;
 import org.junit.Test;
 

sql/hive/src/test/java/org/apache/spark/sql/hive/JavaMetastoreDataSourcesSuite.java

@@ -27,7 +27,6 @@
 import org.apache.hadoop.fs.FileSystem;
 import org.apache.hadoop.fs.Path;
 import org.junit.After;
-import org.junit.Assert;
 import org.junit.Before;
 import org.junit.Test;