removed references to obsolete ckan.model.ResourceGroup from reports.py

This change removes three lines from reports.py which reference the now deleted `ckan.models.ResourceGroup`. This gets ckanext-qa to work with CKAN 2.3a (source install of master branch on Ubuntu 14.10 LTS on Amazon EC2 t2.medium). 

Two tests fail - likely more to do with my test setup.
```
(default)user@machine:/usr/lib/ckan/default/src# nosetests --ckan ../ckanext-qa/tests/
# throws
FAIL: test_extension.TestQAController.test_package_openness_scores
AssertionError: PATH_INFO doesn't start with /: 'qa_dataset_action'
# and
FAIL: test_extension.TestQAController.test_packages_with_broken_resource_links
AssertionError: PATH_INFO doesn't start with /: 'qa_dataset_action'
```

However, both `paster qa update` and the GUI links work.
```
# with CKAN's virtualenv activated
(default)user@machine:/usr/lib/ckan/default/src# paster --plugin=ckanext-qa qa update -c /path/to/my.ini

# GUI
/qa
/qa/dataset/five_stars
/qa/dataset/broken_resource_links
/api/2/util/qa/broken_resource_links_by_dataset.csv
/qa/organisation/broken_resource_links
/api/2/util/qa/organisations_with_broken_resource_links/all.csv
```

Before this pull request is merged, someone with a better understanding of the purpose of ResourceGroup should make sure that removing it from this extension doesn't violate assumptions or business logic I'm not aware of.