libvpx

Remove redundant mv_pred use for sub8x8 blocks

The sub8x8 blocks has its own motion vector reference scheme. The
mv_pred is only used blocks of sizes 8x8 and above, to find the
starting point for motion search.

This change does not change any coding behavior. It makes the
encoding process slightly faster. (0.5% speed-up for local test on
speed 1.)

Change-Id: I746ee6ef0eac19aa3621be014afa12be8d82cbb9