Remove FF zoekt_syntax_transpile

What does this MR do and why?

This change removes a feature flag called "zoekt_syntax_transpile" that was controlling whether search query syntax gets converted when using the API. Previously, the system would only perform this syntax conversion if the feature flag was enabled. Now, the syntax conversion will always happen for API requests, meaning the feature has been fully rolled out and no longer needs to be toggled on or off. The associated test that checked the disabled state of this feature flag was also removed since it's no longer relevant.

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Related to #553058 (closed)