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It has been replaced by the 'formations' key within the 'app.json' file format.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

@@ -0,0 +1,6 @@
+function worker() {
+  console.log('sleeping for 60 seconds');
+  setTimeout(worker, 60 * 1000);
+}
+
+worker();