remove vp9_dthread.c

and the related prototypes in vp9_dthread.h. the last references were
removed in:
09dabc58d VP9_COMMON: rm frame_parallel_decode

vp9_dx_iface.c still uses FrameWorkerData

BUG=webm:1395

Change-Id: Ica8e98ae776fc0105f1fbbed9e0a729808980810
3 files changed