SPARK-6433 omit "jar" goal from jar plugin

Author: steveloughran

pom.xml

@@ -1491,13 +1491,6 @@
           </archive>
         </configuration>
         <executions>
-          <execution>
-            <id>prepare-jar</id>
-            <phase>prepare-package</phase>
-            <goals>
-              <goal>jar</goal>
-            </goals>
-          </execution>
           <execution>
             <id>prepare-test-jar</id>
             <phase>prepare-package</phase>