remove Sys_utils.sleep
remove Sys_utils.sleep

Summary: this is unused, but is obsoleted by Unix.sleepf anyway

Reviewed By: SamChou19815

Differential Revision: D38229979

fbshipit-source-id: 6f89649549104c46bb93543f48cecece3d0f2a27

mroch/flow,facebook/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,facebook/flow,mroch/flow,facebook/flow