drm/amdgpu: drop ras self test

this function is not needed any more. error injection is
the only way to validate ras but it can't be executed in
amdgpu_ras_init, where gpu is even not initialized

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>