Revert "Don't reject invalid warped motion model"
This reverts commit 1eac584f. Reason for revert: As Peter pointed out, reverting motion_mode to SIMPLE_TRANSLATION only at the decoder side could mess up coding of compound_type. We'll fix this issue from encoder side. Change-Id: Ic9b3b8e0f0d06b22630d5f870a8d9737a596fa22