flush_proc: unnecessary code

[akiradeveloper]

I think we don't need to wait_for_flushing here because it's given in this context.

    /*
     * Deferred ACK for barrier requests
     * To serialize barrier ACK in logging we wait for the previous segment
     * to be persistently written (if needed).
     */
    wait_for_flushing(wb, SUB_ID(seg->id, 1)); // FIXME remove this!
    process_deferred_barriers(wb, job);