Fix the test getting stuck by adding default constructor.

Remove comment mentioning roaring bitmaps in KryoSerializerSuite.

core/src/main/scala/org/apache/spark/util/collection/BitSet.scala

@@ -31,6 +31,8 @@ class BitSet(private[this] var numBits: Int) extends Externalizable {
   private var words = new Array[Long](bit2words(numBits))
   private def numWords = words.length
 
+  def this() = this(0)
+
   /**
    * Compute the capacity (number of bits) that can be represented
    * by this bitset.

core/src/test/scala/org/apache/spark/serializer/KryoSerializerSuite.scala

@@ -322,12 +322,6 @@ class KryoSerializerSuite extends SparkFunSuite with SharedSparkContext {
     val conf = new SparkConf(false)
     conf.set("spark.kryo.registrationRequired", "true")
 
-    // these cases require knowing the internals of RoaringBitmap a little.  Blocks span 2^16
-    // values, and they use a bitmap (dense) if they have more than 4096 values, and an
-    // array (sparse) if they use less.  So we just create two cases, one sparse and one dense.
-    // and we use a roaring bitmap for the empty blocks, so we trigger the dense case w/ mostly
-    // empty blocks
-
     val ser = new KryoSerializer(conf).newInstance()
     val denseBlockSizes = new Array[Long](5000)
     val sparseBlockSizes = Array[Long](0L, 1L, 0L, 2L)