Remove the use of prev_frame_mvs for mfmv
Remove the use of prev_frame_mvs for mfmv

The experiment of mfmv has already extensively leveraged the use of
the motion vectors from the reference frames, hence there is no need
to further add the motion vectors from the previous frame to serve
as the motion vector reference candidates, let alone such motion
vectors have not taken into account of the use of frame offsets.

Experimental results collected so far have shown this patch has zero
coding performance impact.

Change-Id: I57a2f8b60797bd5e8f0a95206689b6e443b28ed1

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom