vpx_thread.h: remove unused sched_yield()
vpx_thread.h: remove unused sched_yield()

usage was removed with:
c1b024b48 Modify map read/write to sync logic in row_mt case

Change-Id: I515fe397083079a4f11702e67c322fd04bdcf410

webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx