Remove civicrm_drupal_create_user

This is a follow-up from: CRM-3538

I recently came across the civicrm_drupal_create_user function, in the api.php file.

This function would create a Drupal user by writing directly to the database, which is a terrible idea. This is what the Drupal APIs are for.

I've checked all the other code in CiviCRM and this function is not called from anywhere.

Given these two points, we should remove it from the code base completely.

I'll put together a PR against master.