Remove Heroku buildpack config; allow devDependencies to be pruned (#3115)

The buildpack config doesn't seem to be needed. This would affect review apps, staging, and self-hosting.

Also, devDependencies are installed by default during the build: https://devcenter.heroku.com/changelog-items/1376

By removing `NPM_CONFIG_PRODUCTION=false` we allow these to be pruned so they're kept out of the slug. This also has the advantage of creating a test environment in PRs where missing production dependencies will cause things to break. That'd be a good thing!

.buildpacks

@@ -1,2 +0,0 @@
-https://github.com/mojodna/heroku-buildpack-cairo.git
-https://github.com/heroku/heroku-buildpack-nodejs.git

app.json

@@ -6,10 +6,6 @@
   "repository": "https://github.com/badges/shields",
   "logo": "http://shields.io/favicon.png",
   "env": {
-    "NPM_CONFIG_PRODUCTION": {
-      "description": "Configure whether devDependencies are installed (they are needed for the build).",
-      "value": "false"
-    },
     "WHEELMAP_TOKEN": {
       "description": "Configure the token to be used for the Wheelmap service.",
       "required": false