drm/amdgpu: nuke the VM PD/PT shadow handling

This was only used as workaround for recovering the page tables after
VRAM was lost and is no longer necessary after the function
amdgpu_vm_bo_reset_state_machine() started to do the same.

Compute never used shadows either, so the only proplematic case left is
SVM and that is most likely not recoverable in any way when VRAM is
lost.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>