chore(migration): remove unused cert

Author: duanhongyi

rootfs/api/migrations/0013_migration_permissions_and_certificates.py

@@ -4,6 +4,7 @@
 from guardian.shortcuts import assign_perm, get_users_with_perms, remove_perm
 from api.models.app import App, VIEW_APP_PERMISSION, CHANGE_APP_PERMISSION
 from api.models.domain import Domain
+from api.models.certificate import Certificate
 
 
 def migration_permission(apps, schema_editor):
@@ -27,6 +28,7 @@ def migration_certificate(apps, schema_editor):
                 certificate.save()
                 domain.certificate = certificate
                 domain.save()
+    Certificate.objects.filter(app=None).delete()
 
 
 class Migration(migrations.Migration):