[SPARK-5993][Streaming][Build] Fix assembly jar location of kafka-assembly

Published Kafka-assembly JAR was empty in 1.3.0-RC1
This is because the maven build generated two Jars-
1. an empty JAR file (since kafka-assembly has no code of its own)
2. a assembly JAR file containing everything in a different location as 1
The maven publishing plugin uploaded 1 and not 2.
Instead if 2 is not configure to generate in a different location, there is only 1 jar containing everything, which gets published.

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #4753 from tdas/SPARK-5993 and squashes the following commits:

c390db8 [Tathagata Das] Fix assembly jar location of kafka-assembly

external/kafka-assembly/pom.xml

@@ -33,9 +33,6 @@
 
   <properties>
     <sbt.project.name>streaming-kafka-assembly</sbt.project.name>
-    <spark.jar.dir>scala-${scala.binary.version}</spark.jar.dir>
-    <spark.jar.basename>spark-streaming-kafka-assembly-${project.version}.jar</spark.jar.basename>
-    <spark.jar>${project.build.directory}/${spark.jar.dir}/${spark.jar.basename}</spark.jar>
   </properties>
 
   <dependencies>
@@ -61,7 +58,6 @@
       <artifactId>maven-shade-plugin</artifactId>
       <configuration>
         <shadedArtifactAttached>false</shadedArtifactAttached>
-        <outputFile>${spark.jar}</outputFile>
         <artifactSet>
           <includes>
             <include>*:*</include>