lvpoll: don't use hints

lvpoll: don't use hints

There's a bug when lvpoll attempts to write new hints,
related to the fact that lvpoll does not follow the same
scanning process as standard commands.
Fix by disabling the use of hints in lvpoll.  We may want
to renable hints in lvpoll in a way that they can be used,
if valid, but not updated if they don't exist or are invalid.