deleting secret_references from computed agent policy

[juliaElastic]

What is the problem this PR solves?

Cleanup

How does this PR solve the problem?

Remove secret_references from the computed agent policy being sent out to agents.

How to test this PR locally

Add an integration with secrets, enroll an agent, check in agent diagnostics that the agent policy doesn't contain secret_references

Design Checklist

• I have ensured my design is stateless and will work when multiple fleet-server instances are behind a load balancer.
• I have or intend to scale test my changes, ensuring it will work reliably with 100K+ agents connected.
• I have included fail safe mechanisms to limit the load on fleet-server: rate limiting, circuit breakers, caching, load shedding, etc.

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool

Related issues

Related to elastic/kibana#154715

[elasticmachine]

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• Start Time: 2023-08-15T09:33:45.729+0000

• Duration: 39 min 49 sec

Test stats 🧪

Test	Results
Failed	0
Passed	752
Skipped	1
Total	753

💚 Flaky test report

Tests succeeded.

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