Removing redundant call of vp9_init_mbmode_probs().

This function is called from vp9_setup_past_independence() which is called
before the modified piece of code. Moving reset of inter_mode_probs  into
vp9_init_mbmode_probs() for consistency.

Change-Id: Ib188e8798e1fbe15407fd501406761b746fdda95