Prevent duplicates on DiscoveryModule (bug 1013368)

See https://bugzilla.mozilla.org/show_bug.cgi?id=1013368
> Note that .delete() against a queryset doesn't call the Model.delete()

We don't have any custom `delete`. Signals are sent the same, right?

> isn't it possible to .filter(module_in=delete) to delete all module within the same time?

Much better, thanks! :)