fix: use MailmanClient().unsubscribe in ModalSubscribeView

Author: herzog0

mailing_list/views.py

@@ -566,7 +566,7 @@ def _handle_authenticated(self, request, email, list_ids, managed_lists):
                 sub.delete()
             else:
                 try:
-                    mailman_unsubscribe(sub.email, lid)
+                    MailmanClient().unsubscribe(sub.email, lid)
                     UserMailingListSubscription.objects.filter(
                         user=request.user, list_id=lid
                     ).delete()