Remove client jar

This reverts building a separate client jar for xpack. It is not
necessary because we already build x-pack as a jar and publish to
elastic maven, since extension authors need that.

Original commit: elastic/x-pack-elasticsearch@2fab06b42c

elasticsearch/x-pack/build.gradle

@@ -9,7 +9,6 @@ esplugin {
   name 'x-pack'
   description 'Elasticsearch Expanded Pack Plugin'
   classname 'org.elasticsearch.xpack.XPackPlugin'
-  hasClientJar = true
 }
 
 ext.versions = [