acl: a deleted ACL policy still shows when reading token it was assigned to

### Nomad version
`Nomad v1.3.3 (428b2cd8014c48ee9eae23f02712b7219da16d30)`

### Operating system and Environment details
Tested locally on macOS using a development agent.

### Issue
When an ACL policy is deleted that is referenced by an active ACL token, the ACL token CLI and API still details the linked policy.

### Reproduction steps
1. Run a development agent using the following config snippet to ensure ACLs are enabled:
```
acl {
enabled = true
}
```

2. Bootstrap the ACL system via the `nomad acl boostrap` command.

3. Create an ACL policy using the example policy below via the `nomad acl policy apply -token= test `:
```
node {
policy = "read"
}
```

4. Create an ACL token referencing the policy created above via `nomad acl token create -policy=test`
5. Delete the policy via `nomad acl policy delete test`
6. Read the information regarding the created token using `nomad acl token -token= self`

#### Expected Result
The final read of the token shows no policies linked.

#### Actual Result
The delete policy still shows, which can be a little confusing when trying to understand why the token no longer works.

Reproduce the behavior with the documented ACL policy and token commands, then trace the ACL token self/read path used by the CLI and API. The fix is complete when a token referencing a deleted policy shows no linked policies, while the existing reproduction still works for active policies.

Written by the indexing model from the issue text.