Remove redundant intra mode penalty from vp9_pick_inter_mode

The intra mode penalty is covered by intra_cost_penalty. This
commit removes the other intra cost threshold, provided that the
constant 50 is negligible in normal rate-distortion cost.

Change-Id: I9b8b7483c43b9a41741622e7057def1f7d51bb72
Merge "Remove redundant intra mode penalty from vp9_pick_inter_mode"