Revert "Add the ability to exclude remote users in user directory search results (#18300)"

[3nprob]

This reverts commit fe8bb62.

It introduced a regression in the workers test suite.

• Part of CI test suites are flaky #18405

Pull Request Checklist

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• Pull request includes removes a changelog file. The entry should:
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  (run the linters)

[anoadragon453]

@3nprob could you please detail what the regression was?

[3nprob]

@3nprob could you please detail what the regression was?

https://github.com/element-hq/synapse/actions/runs/14797357096/job/41547797957
https://github.com/element-hq/synapse/actions/runs/14856945196/job/41712653185

Also linked #18405 in PR description.

I see that a release has just been cut including the change-under-revert here... Does that complicate things further?

[anoadragon453]

@3nprob we have a separate issue with Sytest flaking, which has been ongoing for a while. Re-running the first job you posted shows the tests passing: https://github.com/element-hq/synapse/actions/runs/14797357096

I'd be happy to have someone dig into why that is happening, but I don't believe #18300 is the cause.