Stop using heroku-buildpack-nodejs (#14341)

heroku-buildpack-ruby started to install Node 12 by default.
This makes it possible to run Mastodon without heroku-buildpack-nodejs.

app.json

@@ -88,9 +88,6 @@
     {
       "url": "https://github.com/heroku/heroku-buildpack-apt"
     },
-    {
-      "url": "heroku/nodejs"
-    },
     {
       "url": "heroku/ruby"
     }