Fix a handful of misc. IntelliJ inspections

Author: JoshRosen

core/src/main/java/org/apache/spark/shuffle/unsafe/UnsafeShuffleExternalSorter.java

@@ -139,7 +139,7 @@ private SpillInfo writeSpillFile() throws IOException {
 
     // Currently, we need to open a new DiskBlockObjectWriter for each partition; we can avoid this
     // after SPARK-5581 is fixed.
-    BlockObjectWriter writer = null;
+    BlockObjectWriter writer;
 
     // Small writes to DiskBlockObjectWriter will be fairly inefficient. Since there doesn't seem to
     // be an API to directly transfer bytes from managed memory to the disk writer, we buffer
@@ -202,7 +202,6 @@ private SpillInfo writeSpillFile() throws IOException {
           writeBuffer,
           PlatformDependent.BYTE_ARRAY_OFFSET,
           toTransfer);
-        assert (writer != null);  // To suppress an IntelliJ warning
         writer.write(writeBuffer, 0, toTransfer);
         recordReadPosition += toTransfer;
         dataRemaining -= toTransfer;

core/src/main/java/org/apache/spark/shuffle/unsafe/UnsafeShuffleSorter.java

@@ -17,10 +17,8 @@
 
 package org.apache.spark.shuffle.unsafe;
 
-import java.io.IOException;
 import java.util.Comparator;
 
-import org.apache.spark.unsafe.memory.MemoryBlock;
 import org.apache.spark.util.collection.Sorter;
 
 final class UnsafeShuffleSorter {
@@ -71,7 +69,7 @@ public long getMemoryUsage() {
    * @param partitionId the partition id, which must be less than or equal to
    *                    {@link PackedRecordPointer#MAXIMUM_PARTITION_ID}.
    */
-  public void insertRecord(long recordPointer, int partitionId) throws IOException {
+  public void insertRecord(long recordPointer, int partitionId) {
     if (!hasSpaceForAnotherRecord()) {
       expandSortBuffer();
     }

core/src/main/java/org/apache/spark/shuffle/unsafe/UnsafeShuffleWriter.java

@@ -135,7 +135,7 @@ public void write(scala.collection.Iterator<Product2<K, V>> records) throws IOEx
       closeAndWriteOutput();
     } catch (Exception e) {
       // Unfortunately, we have to catch Exception here in order to ensure proper cleanup after
-      // errors becuase Spark's Scala code, or users' custom Serializers, might throw arbitrary
+      // errors because Spark's Scala code, or users' custom Serializers, might throw arbitrary
       // unchecked exceptions.
       try {
         sorter.cleanupAfterError();