[9.4](backport #7468) Remove service_token from secret_path for remote ES

[mergify]

What is the problem this PR solves?

We delete the service_token from the parsed policy when output is of remoteES type. See. But we do not remove the reference for it from secret_path.output.[OuputID].service_token which causes constant stream of errors on elastic-agent side when attempting to read the nil service_token

The error logged is

failed to add secret markers: secret path outputs.OUTPUT_ID.service_token does not exist

How does this PR solve the problem?

This PR removes the corresponding service_token from secret_path before sending the policy to elastic-agent.

How to test this PR locally

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

• Closes Agents logging constant error: "failed to add secret markers: secret path outputs.OUTPUT_ID.service_token does not exist" elastic-agent#15798
• Closes "Unable to find secret path" warning is displayed while collecting diagnostics when Remote ES is selected. #5664

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This is an automatic backport of pull request #7468 done by [Mergify](https://mergify.com).