Remove the use of use_lastframe_partitioning at speed 4

Remove the use of use_lastframe_partitioning at speed 4

The use of use_lastframe_partitioning is totally removed in good-
quality encoding. Its usage in real-time encoding needs to be
evaluated to see if it can be removed too.

The Borg tests at speed 4 showed:
stdhd set: 0.220% psnr gain, 0.166% ssim gain;
derf set:  0.329% psnr gain, 0.476% ssim gain.

Speed test on selected clips showed 1.54% speedup.(Worst case:
pedestrian_area_1080p25.y4m, speed loss: 1.5%)

Change-Id: I1c844d329b0b5678558439b887297c1be7ddab00