(Drush 6) Allow overwriting an existing site when --no-core is specified.

This fixes the bug reported in [this Drupal.org issue](https://www.drupal.org/node/1673676). When specifying --no-core, you should be able to overwrite an existing site directory.

I've also created this pull request for Drush 7 [here](https://github.com/drush-ops/drush/pull/920).