DeleteView uses POST (not DELETE) and its handling should therefore be moved to form_valid()

[torgeirl]

Django warning: DeleteViewCustomDeleteWarning: DeleteView uses FormMixin to handle POST requests. As a consequence, any custom deletion logic in GroupCommentDeleteView.delete() handler should be moved to form_valid()

[Levijatan]

Fixed this in 6ee45c5. Moved logic to form_valid() and overwrote delete() with delete() from grandparent since delete() was changed in parent DeleteView from cradmin_legacy.