drm/ttm: drop unbind callback.

The drivers now control this, so drop unbinding.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201020010319.1692445-5-airlied@gmail.com