This was only used to detect userspace including the same bo multiple
times in a submit.  But ww_mutex can already tell us this.

When we drop struct_mutex around the submit ioctl, we'd otherwise need
to lock the bo before adding it to the bo_list.  But since ww_mutex can
already tell us this, it is simpler just to remove the bo_list.
Signed-off-by: Rob Clark <robdclark@chromium.org>
Link: https://lore.kernel.org/r/20210728010632.2633470-11-robdclark@gmail.comSigned-off-by: Rob Clark <robdclark@chromium.org>