Remove drush site install from php-fpm entrypoint (move to Tekton task)

There is logic in the php-fpm entrypoint to install the site. Even though it first checks if the site is already installed, that's the wrong place for such an action: https://gitlab.cern.ch/drupal/paas/drupal-runtime/-/blob/master/images/php-fpm/run-php-fpm.sh#L2-35

We should turn it into a Tekton task and then remove it.

This code is never executed anyway, because the env var ENVIRONMENT isn't set. This means it's safe to just remove.