Drop use_sync_service_token_worker FF

What does this MR do and why?

This MR removes the use_sync_service_token_worker FF that has been enabled by default since 16.9.

I also replaced several occurrences of the now deprecated Ai::SyncServiceTokenWorker with CloudConnector::SyncServiceTokenWorker. These classes are identical in anything but name so we should start calling into the CloudConnector:: version everywhere.

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Related to #431608