Remove deprecated tree coding for the global motion type.
Encoding with literals costs at most 0.8 bit per frame or
tile group for the identity type, and saves bits for other
types.

AWCY (objective-1-fast): PSNR +0.02%, PSNR-HVS/MSSSIM +0.03%.

Change-Id: I421a39fd8f830b2e40d105ea3c3688099a2b6d15