[fix] Updated #576

Fixes #576

openwisp_monitoring/device/base/models.py

@@ -9,7 +9,7 @@
 from django.core.cache import cache
 from django.core.exceptions import ValidationError
 from django.db import models
-from django.db.models.signals import post_delete, post_save
+from django.db.models.signals import post_delete
 from django.dispatch import receiver
 from django.utils.timezone import now
 from django.utils.translation import gettext_lazy as _