## Bug description
Feature Flag cannot recreate with the same key after deleting 

Steps to reproduce: 

1. create feature flag with key `bla`
2. delete the feature flag
3. try to create again feature flag with `bla` key
4. alert will popup with This key already exists
![image](https://user-images.githubusercontent.com/24251369/99444738-4e5a4680-2925-11eb-822a-5a9397fba965.png)


## Expected behavior
New feature flag will be added

## Environment

self-hosting 



#### *Thank you* for your bug report – we love squashing them!

posthog/api/feature_flag.py
created_by = UserSerializer(required=False, read_only=True)
    is_simple_flag = serializers.SerializerMethodField()
    class Meta:
        model = FeatureFlag
        fields = [
            "id",
            "name",
            "key",
            "rollout_percentage",
            "filters",
            "deleted",
            "active",
            "created_by",
            "created_at",
            "is_simple_flag",
        ]
    
    
    def get_is_simple_flag(self, feature_flag: FeatureFlag):
        filters = feature_flag.filters
        if not filters:
            return True
        if not filters.get("properties", []):
            return True
        return False
    def create(self, validated_data: Dict, *args: Any, **kwargs: Any) -> FeatureFlag: