Correct Preconditions error message in WorkerFactory

[phlego]

What was changed

This PR correct the error message for the start method of WorkerFactory.
The fix correctly shows State.Initial and State.Started as the expected states.
The PR also remove a unused variable and several unused imports.

Why?

The original code had State.Initial.name() duplicated in the error message, which was likely a copy-paste error.

Checklist

1. Closes
   N/A

2. How was this tested:
   N/A

3. Any docs updates needed?
   No

[CLAassistant]

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[CLAassistant]

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[phlego]

Hi @Quinn-With-Two-Ns
I'm not sure the integration tests are flaky or something is wrong with my PR.
It seems that you helped rerun the pipeline once a couple days ago.
Wonder if you could help me out?

[Quinn-With-Two-Ns]

Hi @phlego thank you for the contribution, please see these instructions #2686 (comment) to fix CI thanks

[phlego]

Done @Quinn-With-Two-Ns
Thanks a lot for your support 🙏