Remove post install .env copy (roots#595)

`.env.example` is just an example and doesn't contain useful defaults.
Copying this file can be confusing since it needs to be edited
regardless. By not copying this file, we'll surface errors of missing
config values sooner since they'll be no default values.

composer.json

@@ -65,9 +65,6 @@
     "wordpress-install-dir": "web/wp"
   },
   "scripts": {
-    "post-root-package-install": [
-      "php -r \"copy('.env.example', '.env');\""
-    ],
     "test": [
       "phpcs"
     ]