Get rid of _param:influxdb_user

The StackLight username for InfluxDB is set to 'lma' by default. If one
wants to override it, it should be done in the user model.
diff --git a/heka/aggregator/single.yml b/heka/aggregator/single.yml
index 4bef48c..123538b 100644
--- a/heka/aggregator/single.yml
+++ b/heka/aggregator/single.yml
@@ -10,7 +10,7 @@
       influxdb_host: ${_param:heka_influxdb_host}
       influxdb_port: ${_param:influxdb_port}
       influxdb_database: ${_param:influxdb_database}
-      influxdb_username: ${_param:influxdb_user}
+      influxdb_username: lma
       influxdb_password: ${_param:influxdb_stacklight_password}
       nagios_host: ${_param:nagios_host}
       nagios_username: ${_param:nagios_username}