Remove the drupal:install Composer script in favor of a normal Drush site:install

The prototype had a drupal:install script in its template composer.json, which was intended to install Drupal CMS along with any necessary customizations.

Since Drupal CMS has a custom install profile, we really don't need many of those tweaks anymore. That means the drupal:install script is basically cruft, and can go away -- drush site:install covers all the bases.