Now the single ref comp mode should work with WEDGE and
COMPOUND_SEGMENT. For motion_var, the OBMC_CAUSAL mode uses the 2nd
predictor if the neighboring block is single ref comp mode predicted.

This patch removes the mode of SR_NEAREST_NEWMV and leaves four
single ref comp modes in total:

SR_NEAREST_NEARMV
SR_NEAR_NEWMV
SR_ZERO_NEWMV
SR_NEW_NEWMV

Change-Id: If6140455771f0f1a3b947766eccf82f23cc6b67a