drm/amdgpu: remove AMDGPU_GEM_CREATE_SHADOW flag

Remove unused AMDGPU_GEM_CREATE_SHADOW flag.
Userspace is never allowed to use this.

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>