The speedup in rd_pick_partition() function makes it possible
to drop use_lastframe_partitioning feature. By doing that, we
achieve good PSNR gain with small speed loss. Also, this makes
encoding loop less complicated. The code cleanup patch will
follow.

Borg tests showed:
1. At speed 2,
   stdhd set: 0.201% PSNR gain, 0.133% SSIM gain;
   derf set:  0.262% PSNR gain, 0.276% SSIM gain.
2. At speed 3,
   stdhd set: 0.139% PSNR gain, 0.109% SSIM gain;
   derf set:  0.447% PSNR gain, 0.442% SSIM gain.

The average speed loss over selected test clips is within 1%
with the worst case of 4%.

Change-Id: Icfd2ded7869372b585a6972855d933b3d0280d90