Delete obsolete comment about the -thread flag
Delete obsolete comment about the -thread flag

The -thread flag is not necessary for Lwt_preemptive since Lwt 4.0.0.

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c-cube/lwt,ocsigen/lwt,rneswold/lwt,c-cube/lwt,ocsigen/lwt,rneswold/lwt,ocsigen/lwt,rneswold/lwt,c-cube/lwt