[Fleet] Cannot delete a managed agent policy (#90505)
## Summary
Managed policy cannot be deleted via API or UI
closes https://github.com/elastic/kibana/issues/90448
### Checklist
- [x] [Unit or functional tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html) were updated or added to match the most common scenarios
#### Manual testing
<details><summary>UI screenshot</summary>
<img width="1522" alt="Screen Shot 2021-02-05 at 1 56 13 PM" src="https://user-images.githubusercontent.com/57655/107076955-2ce95800-67ba-11eb-8bd4-d2ba3182e516.png">
</details>
<details><summary>API commands</summary>
```
## Create a managed policy
curl --user elastic:changeme -X POST localhost:5601/api/fleet/agent_policies -H 'Content-Type: application/json' -d'{ "name": "User created MANAGED", "namespace": "default", "is_managed": true}' -H 'kbn-xsrf: true'
{"item":{"id":"17ebd160-67ee-11eb-adb2-f16c6e20580c","name":"User created MANAGED","namespace":"default","is_managed":true,"revision":1,"updated_at":"2021-02-05T20:09:46.614Z","updated_by":"elastic"}}
## Cannot delete it
curl --user elastic:changeme -X POST 'http://localhost:5601/api/fleet/agent_policies/delete' -H 'kbn-xsrf: abc' -H 'Content-Type: application/json' --data-raw '{"agentPolicyId": "17ebd160-67ee-11eb-adb2-f16c6e20580c" }'
{
"statusCode": 400,
"error": "Bad Request",
"message": "Cannot delete managed policy 17ebd160-67ee-11eb-adb2-f16c6e20580c"
}
## Set policy to unmanaged
curl --user elastic:changeme -X PUT localhost:5601/api/fleet/agent_policies/17ebd160-67ee-11eb-adb2-f16c6e20580c -H 'Content-Type: application/json' -d'{ "name": "User created MANAGED", "namespace": "default", "is_managed": false}' -H 'kbn-xsrf: true'
{
"item": {
"id": "17ebd160-67ee-11eb-adb2-f16c6e20580c",
"name": "User created MANAGED",
"namespace": "default",
"is_managed": false,
"revision": 3,
"updated_at": "2021-02-05T20:10:45.383Z",
"updated_by": "elastic",
"package_policies": []
}
}
## Can delete
curl --user elastic:changeme -X POST 'http://localhost:5601/api/fleet/agent_policies/delete' -H 'kbn-xsrf: abc' -H 'Content-Type: application/json' --data-raw '{"agentPolicyId": "17ebd160-67ee-11eb-adb2-f16c6e20580c" }'
{
"id": "17ebd160-67ee-11eb-adb2-f16c6e20580c",
"name": "User created MANAGED"
}
```
</details>
bda7b2816f
