drm/amdgpu: remove unused code

Remove unused code related to shadow BO.

v2: removing shadow bo ptr from base class.

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>