drm/amdgpu: remove sync_to from sync obj v2

Not needed any more without semaphores.

v2: remove unused variables as well

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>