Remove redundant reference frame threshold settings

When a reference frame type is not in the frame buffer, the mode
search threshold will be set to INT_MAX, so as to effectively
turn off the mode entries in the rate-distortion optimization loop
that involves this reference frame type. This operation is now
integrated in the ref_frame_skip_mask scheme. This commit hence
removes the redundant mode search threshold setting.

Change-Id: Ib18f45da611afda2af275201efd367df7f5101ab