managed: fix ManagedSerializer interfering with other serializers

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

authentik/managed/api.py

@@ -1,10 +1,8 @@
 """Serializer mixin for managed models"""
 from rest_framework.fields import CharField
 
-from authentik.core.api.utils import PassiveSerializer
 
-
-class ManagedSerializer(PassiveSerializer):
+class ManagedSerializer:
     """Managed Serializer"""
 
     managed = CharField(read_only=True, allow_null=True)