Update collectd decoder for Elasticsearch metrics

The Elasticsearch metrics from collectd now comply with the default
decoding scheme.
diff --git a/heka/files/lua/decoders/collectd.lua b/heka/files/lua/decoders/collectd.lua
index 085821c..8762b54 100644
--- a/heka/files/lua/decoders/collectd.lua
+++ b/heka/files/lua/decoders/collectd.lua
@@ -425,7 +425,7 @@
                 else
                     msg['Fields']['name'] = 'virt' .. sep .. metric_name
                 end
-            elseif metric_source == 'elasticsearch_cluster' or metric_source == 'influxdb' then
+            elseif metric_source == 'influxdb' then
                 msg['Fields']['name'] = metric_source .. sep .. sample['type_instance']
             elseif metric_source == 'http_check' then
                 msg['Fields']['name'] = metric_source