Remove unused scheduler driver methods

Commits 8466013fbed8b0ec7bb64c6eb183c9bb1b846665 and
cd16fa7b485e53f611a3f831569ddacce5393368 remove the last uses of
instance_update_db() and handle_schedule_error().

Change-Id: Ia1f1075f52516c0cdff4b0d1fc5f0a8738d0ee1d