[Streaming][Minor] Remove useless type signature of Java Kafka direct stream API

cc tdas .

Author: Saisai Shao <saisai.shao@intel.com>

Closes #4817 from jerryshao/signature-minor-fix and squashes the following commits:

eebfaac [Saisai Shao] Remove useless type parameter

external/kafka/src/main/scala/org/apache/spark/streaming/kafka/KafkaUtils.scala

@@ -512,7 +512,7 @@ object KafkaUtils {
    * @param topics Names of the topics to consume
    */
   @Experimental
-  def createDirectStream[K, V, KD <: Decoder[K], VD <: Decoder[V], R](
+  def createDirectStream[K, V, KD <: Decoder[K], VD <: Decoder[V]](
       jssc: JavaStreamingContext,
       keyClass: Class[K],
       valueClass: Class[V],