vp9_dx: rm worker thread creation
vp9_dx: rm worker thread creation

creating a thread associated with the sole worker isn't necessary when
only execute() is being used after the removal of frame_parallel_decode.

BUG=webm:1395

Change-Id: I2255ce72607321e5708bc82a632dc6825d4eff5c

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