Delete voucher even if it is contained in carts

Fix PRETIXEU-R1

src/pretix/control/views/vouchers.py

@@ -140,6 +140,7 @@ class VoucherDelete(EventPermissionRequiredMixin, DeleteView):
             messages.error(request, _('A voucher can not be deleted if it already has been redeemed.'))
         else:
             self.object.log_action('pretix.voucher.deleted', user=self.request.user)
+            self.object.cartposition_set.all().delete()
             self.object.delete()
             messages.success(request, _('The selected voucher has been deleted.'))
         return HttpResponseRedirect(success_url)