Removed read_mv_ref

Decode the mv mode with if-then-elses instead of traversing
the vp8_mv_ref_tree data structure.  This will make it
easier to interleave vp8_find_near_mvs and vp8_mv_ref_probs.

Change-Id: I1e798d6ec40fcaeeff06ccc82f81201978d12f74