Simple: removed conflicting score field from raw result objects

Referring both [Pull Request #721](https://github.com/toastdriven/django-haystack/pull/721) which refers [Issue #482](https://github.com/toastdriven/django-haystack/issues/482)

This keeps consistency with the Solr backend, which resolves this conflict
in the same manner in order to avoid an exception being thrown.

Also removed the redundant mock search index from the simple backend's test suite