autoscaling policies should not be returned for missing jobs

[cgbaker]

Nomad version

0.11.0

Issue

When a job is stopped, its policies are still return from the /v1/scaling/policies endpoint. This doesn't seem to serve any use case; furthermore, it can make it more difficult for external autoscalers to keep track of policies that should be considered.

Instead, policies should be understood to be deleted when the associated job is stopped (instead of only happening when the job is ultimately deleted).

[github-actions]

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