refactor: remove support for DOKKU_SCALE file

It has been replaced by the 'formations' key within the 'app.json' file format.

tests/apps/dockerfile-app-json-formations/web.js

@@ -0,0 +1,12 @@
+var express = require('express');
+
+var app = express();
+
+app.get('/', function(request, response) {
+  response.send('nodejs/express');
+});
+
+var port = process.env.PORT || 5000;
+app.listen(port, function() {
+  console.log("Listening on " + port);
+});