client: Remove unused `FakeServer::build_user_store` method (#55444)

This PR removes the `build_user_store` method from the `FakeServer`, as
it was not used anywhere.

Release Notes:

- N/A
1581a08c49Marshall Bowers committed on 5/1/2026, 5:47:16 PM· committed by GitHubparent6b28db5
1 file changedLine totals unavailable