Remove Prometheus pushgateway code

Remove code pushing metrics to a hardcoded Prometheus pushgateway that
is no longer in operation.  There's no point supporting the use of a
pushgateway as there's already a full client implemented and since the
master server is not ephemeral or batch job it shouldn't be using a
pushgateway in the first place.

config.json.dist

@@ -57,7 +57,6 @@
 	"mirrorAllChat": false,
 	"__comment7": "Autosave interval for items in the master storage in milliseconds. Defaults to one minute when set to 0 or not set.",
 	"autosaveInterval": 600000,
-	"disablePrometheusPushgateway": false,
 	"__comment8": "This setting only affects the master. It makes every import for things that arent 'electricity' return a maximum of 0. Effectively disables teleport chests.",
 	"disableImportsOfEverythingExceptElectricity": false
 }

config.json.test

@@ -42,6 +42,5 @@
 	"enableCrossServerShout": true,
 	"mirrorAllChat": false,
 	"autosaveInterval": 600000,
-	"disablePrometheusPushgateway": false,
 	"disableImportsOfEverythingExceptElectricity": false
 }

master.js

@@ -82,13 +82,6 @@ const prometheusPrefix = "clusterio_";
 const Prometheus = require('prom-client');
 const expressPrometheus = require('express-prometheus-request-metrics');
 Prometheus.collectDefaultMetrics({ timeout: 10000 }); // collects RAM usage etc every 10 s
-if(!config.disablePrometheusPushgateway){
-	const pushgateway = new Prometheus.Pushgateway('http://hme.danielv.no:9091');
-	setInterval(() => {
-		registerMoreMetrics();
-		pushgateway.push({ jobName: 'clusterio', groupings: {instance: config.publicIP + ":" + config.masterPort, owner: config.username}}, function(err, resp, body) {})
-	}, 15000)
-}
 
 // collect express request durations ms
 app.use(expressPrometheus(Prometheus));