The Solr index is not updated when User Groups are deleted using the Public API.

Steps to reproduce:

1. Unzip attached "DeleteUserGroupTestBundle.zip" bundle to /src;
2. Add the Bundle to the Kernel (new Test\DeleteUserGroupTestBundle\DeleteUserGroupTestBundle());
3. Create a fresh Solr index:

php ezpublish/console ezplatform:solr_create_index

4. Run the Command:

php ezpublish/console -vvv test:test-delete-usergroup

The Command creates and then deletes a User Group.

Expected behavior: to find no "Test UserGroup" User Group in Solr index.
Actual behavior: the "Test UserGroup" User Group is still in Solr index.

This can be verified with the following query in Solr's admin interface:

select?q=Test+UserGroup&wt=json&indent=true

Note: Solr index is updated properly when deleting User Groups in the eZ Publish admin interface. The problem only occurs using the Public API