Remove sample sorting in warped motion sample selection

The original sample selection process involves finding best 8 sorted
samples according to motion vector difference(MVD) between neighbor
block and current block, and then trimming samples. To reduce the
complexity, use the current block width/height as the MVD threshold,
and trim the samples without sorting.

This gives slightly less gain than the original method.
AWCY result:
         PSNR   PSNR HVS   SSIM
Average  -0.07   -0.13     -0.12
Borg test result:
             avg_psnr ovr_psnr ssim
cam_lowres:  -0.112   -0.112  -0.180
lowres:      -0.068   -0.073  -0.125

Change-Id: Ic2f79a170441d5bcb04ea87dddf490ef7fbba8bc